Well, here's the difference. I am a man. If I married a woman, I could go to any other of the US states or any US federal territory, as well as many other countries, and all of my legal rights and privileges automatically carry over. If I'm married in one state, I'm married in every state. That's what the Constitution guarantees with the Full Faith and Credit clause, and the Privileges and Immunities clauses.
But, if I was 'civil-unionized' [what is the verb form of 'civil union'?] to another man, then as soon as I leave my state, in 47 other states we have no rights whatsoever. So, just because of the gender of my partner, we'd lose our legal rights and protections.
Even the states that allow homosexual unions aren't reciprocal. A 'marriage' in Massachusetts doesn't convert to being a 'Civil Union' in Connecticut or Vermont. Nor does a 'Registered Partnership' in California or New Jersey.
Because they are called different things, they are not portable, and don't have any of the same constitutional protections that 'marriage' gets. That's the difference in terminology.
If conservatives really want a uniform definition of marriage, and think the terms shouldn't really be that important, then I've got something that will make everyone happy. How about a Constitutional Amendment that (1) eliminates the term "marriage" from any secular laws, making "marriage" exclusively a religious term, and (2) converts all of what used to be called 'marriages' to "civil unions", and (3) allows any two consenting adults to register their civil union with the state, and get all state benefits that would otherwise be provided to people who were married (as that term had been used).
Any two consenting adults who want to get a civil union can do so, regardless of race, gender, sexual orientation. I'm not going to get into a debate over whether incest should or should not be allowed, so let's just say not allowed for purposes of this proposal.
If people want to get married, that's between them and their church. The "sanctity of marriage" is thus forever protected against government meddling, since "marriage" no longer has any legal meaning under the law. And gay couples now are treated no differently under the law than anyone else, since everyone 'only' gets a civil union, as far as the law is concerned.
If the big issue is regarding terminology and the sanctity of the term "marriage", then religious folk should be completely in favor of this amendment. It guarantees that they can have the term "marriage" all to themselves, based on whatever their religion defines it to be, without any trouble from those equal protection advocates.
Somehow, I don't think that most opponents of same-sex marriage would go for that option, so the argument really isn't about terminology. It may be about wanting to exclude couple they don't like from moving into their state, but then we run into that pesky Constitution with its Article IV (Full Faith and Credit Clause, Privileges and Immunities Clause) and we're back having to deal with courts enforcing the constitution on everyone. So, the arguments based on terminology and non-portability really don't work in the long run.
The second set of arguments are based on the dubious claim that heterosexual marriage requires special status because of the unique benefits it provides to society -- benefits that would be lost if marriage were allowed to same-sex couples.
The most often touted claim centers around procreation. The argument goes that because homosexuals cannot procreate (get pregnant) in the "old fashioned way" (i.e. without medical assistance), then they shouldn't be married. By that argument, only couples who are capable of biological reproduction should be allowed to married. If one person is sterile, or if the couple refuses to have children, then procreation is not going to happen. But take it one step further. A man and women can currently get married without ever having sex or ever dealing with children. Should the government deny the legal benefits of marriage based on whether people are going to have children? And should the man-woman couple who can't conceive naturally be denied marriage because they can't procreate without medical help? Does that make any sense to anyone?
But what about the argument that the purpose of marriage is to provide a social context for sexual intimacy. That's a non-starter for several reasons. First, is it legal in most US states for two people -- a man and a woman -- to get married and never have sex? Absolutely. Sexual relations are not a requirement to get a marriage license. And before people go point to those cases where a marriage was annulled because of no sexual relations, look them up and read the actual court holding -- the actual legal grounds for rescission were fraudulent inducement based on material misrepresentation of fact. Not lack of sex.
So, two people (man and woman) can get married for the legal and financial benefits without ever having sex. Then let's take two straight men, who will never have sex and will never kiss in public (or private) want to get married, for the legal and tax benefits. The Religious Conservatives shouldn't have a problem, right? I mean, nothing in the Bible says that two men cannot be involved in a financial partnership. Or gain other legal benefits through a government registration process. And we're talking about straight men, not gays, so there's no homosexual relations (aka "perverted sex acts" as some conservative might say). Thus, where no gay sex is involved, marriage between two men should be no problem, right?
Let's ignore the fact that the Supreme Court has already said that homosexual relations in private between consenting adults is none of the government's business. Lawrence v. Texas. That's not going to change, no matter how conservative a court the Republicans get appointed. And the Supreme Court has already said that marriage is a fundamental right, and that right includes choosing your spouse. Loving v. Virginia. And absent some religious/moral issue (for example, taking sex itself out of the equation) that's not going to change either.
So, the perfect test case is two straight people of the same gender, who won't have sex, and just want to get married for the legal benefits. Why is this the perfect test case? Because if conservative still oppose it then they are either arguing that their objection is based solely on gender-based discrimination, since sexual orientation is not involved at all, or their objection is because this would allow those couples who do plan to have sex to also get married, in which case they are trying to limit legal rights based on conduct that is already declared legal, solely because their religion does not approve of this legal conduct. And neither of those are valid arguments in a court of law.
The final attempt at a secular argument is that it is a slippery slope. Once we allow same-sex marriage, what's next? Actually, the arguments supporting same-sex marriage are utterly useless for polygamy, bestiality, or any of the others often mentioned. All the laws relating to marriage rights, spousal benefits, inheritance, community property, etc. -- all are written assuming the legal relationship includes is comprised of two adult humans. Because husbands and wives have exactly the same rights and benefits under the law, gender doesn't matter under these current laws. Allowing same-sex marriage doesn't change anything in terms of how the laws work, since (aside from the actual forms) they are already gender-neutral. The same cannot be said for any other relationships that are not between two adults. Or involving non-humans. The current laws are not structured to convey property to animals, or grant tax or spousal benefits to more than one spouse. And allowing that would require massive re-writing, as opposed to just changing the forms used to record marriages. Thus, while gender (and race) don't matter in terms of how current laws operate, the number of people (and species of non-people) involved do matter. So, there is no slippery slope, because the next step beyond same-sex marriage requires a huge rewrite of all the laws from the ground up. Not just reprinting the forms.
Courts have already looked at these issues, and come to the same conclusion. There are really no valid secular reasons to impose a gender-based limitation on who someone is allowed to marry. But, honestly, those arguments are almost always smoke-screens anyway. The real reason that most people are opposed to same-sex marriage is that they don't like certain behaviors because they are against their religious beliefs. And that brings us back to pure gender-based discrimination based purely on religious prejudice.
The third debate centers around the 'sanctity of marriage'. As an aside, it's always amused me to hear people argue for laws protecting the "sanctity" of marriage, completely forgetting that anything that "sanctity" is an inherently religious issue. Which begs the question why the US/state governments are basing legal rights and statuses on religious definitions.
But. This argument basically asks what should "marriage" mean, from a moral standpoint. Of course, most morals derive from a religious framework. And when the issue is phrased as the 'sanctity of marriage', again that makes it an inherently religious issue. Which means, the vast majority of those opposing homosexual marriage are relying on religious reasons to do so.
Granted, there are some who claim that their objection is not religiously based, it's purely based on their personal moral beliefs. Even if that were true, the argument basically boils down to "I'm heterosexual and married, and I don't like your lifestyle, so I don't want you tarnishing or tainting the concept of marriage by having the same rights I do. So there." It's back to third-grade schoolyard politics -- "we don't like you, so you can't play with us". Now, I have no problem with anyone being opposed to something they don't like. That's free will. But, I do have a problem with prejudice being enacted as law.
Look at the proposed federal amendment define marriage as opposite-sex only. This definition, and the very concept of marriage being promoted by it, is based on religious dogma and personal morality. The Marriage Protection Amendment is designed to enforce one particular religious/moral definition of marriage, just because the people who hold that belief happen to be in the legislative majority at the moment. It's pure might-makes-right majority-rule discrimination. Not to mention that anyone who tries to enforce religious prejudice through laws has forgotten the reason this country was founded in the first place.
Let's ignore, for the moment, the fact that Christianity doesn't actually say that all same-sex relationships are bad. And those who think the Bible doesn't supports the concept of lesbian marriage, haven't read the story of Ruth and Naomi very closely. It's all about the covenant (marriage) between these two women. But trying to get most people to think rationally about religion is like trying to get a herd of cats to push water uphill. It's just not worth the insanity that results from the effort. [And yet here I am, still trying.....]
We've already disposed of the non-religious attempts at justifying the prejudice. We've looked at examples above where "marriage" as a term is removed from the legal realm, to prevent it from being 'tainted' by inclusion. And somehow I can't see the Religious Right allowing two straight men to get married, just for the legal and tax benefits, despite the fact that they relationship does not include any of the homosexual conduct or 'immoral sexual relations' (to use their buzzwords) that they oppose so strongly. Do any of the religious arguments against same-sex marriage even work when sex is taken completely out of the question? Probably not. So, if conservatives are still not satisfied, then their argument isn't really about protecting the "sanctity of marriage" (either as term or as an institution). It's just an attempt to impose their religious definitions as a matter of law.
In fact, every single remaining argument in favor of limiting marriage boils down to a religious disapproval of certain types of relationships. For some reason, the religious majority doesn't see any problem with enforcing their moral beliefs as a matter of law, because they happen to be in the majority. I wonder how they would react if some other religion happend to be in the majority. Actually, we don't have to wonder. Just look at the rage and frustration when religious-based laws are struck down by the courts.
Speaking of which. There's this big cry recently against 'activist judges' making laws. Well, remember that it was 'activist judges' that forced states to allow minorities into schools, and 'activist judges' that forced states to allow people of different races to marry. It's a judge's job to say when laws are illegal. But look at the flipside. In one state, the state legislature allowed gay marriage. However, that state's governor vetoed the law because he said it was "up to the judges to determine what marriage should be". So, even those opposed to homosexual marriage can't seem to get their arguments straight.
The bottom line is that the 'legality' of same sex marriage all comes down to who happens to be in the majority making the laws.
Do we want to be a nation where the laws are based upon on specific interpretation of religious doctrine? Do we want to be a nation where two people lose their rights because they move from one US state to another? Do we want to be a nation where two people have get government permission to fall in love? What happened to "life, liberty, and the pursuit of happiness"?
All the arguments against same-sex marriage that have any substance at all boil down to religious disapproval of certain relationships. And anyone who tries to enforce religious prejudice through laws has forgotten the reason this country was founded in the first place.