If you believe it's immoral to accept privileges that others don't have, have you given away all your money yet? Other people are homeless, starving to death, illiterate, brain damaged, paraplegic. Have you cut off your arms and legs and bashed your skull in yet? Other people are dead; does that make it immoral to fail to commit suicide? WHO THE HELL DO YOU THINK YOU ARE to go around preaching about how other people supposedly shouldn't accept privileges that you haven't been given, when you routinely accept so many other privileges that other people haven't been given? Exactly how ridiculously self-centered and tunnel-visioned has the queer community become, that I've actually seen at least six people seriously argue this on LiveJournal recently while giving not the slightest indication anywhere that it's ever occurred to any of them that THEY have accepted and are continuing to accept more privileges than at least 90% of the world's population possesses? The people I'm hearing complaining about this are Americans with internet access and homes and electricity and clean water and paying jobs and I've never seen any of them posting anything about not being sure whether they can scrape together enough money to feed themselves for the rest of the week. Are you annoyed at a specific opposite-sex couple who are millionaires and have more privileges than you in practically every imaginable way? Fine, but then why don't you complain about all their privileges, instead of just complaining about them having the audacity to get legally married when you and your partner can't? Why don't you ever mention anywhere that you're excluding from your wrath the numerous opposite-sex couples for whom the tax and health insurance benefits of being able to get legally married don't half make up for the economic disadvantages they have that you don't have?
And then there's the fact that the rights accorded by legal marriage do not necessarily benefit both partners equally. In jurisdictions where the divorce laws provide for very equitable distribution of property between ex-spouses, whichever spouse belongs to a group that tends to be more economically discriminated against in employment (women, nonwhite people, disabled people, etc.) may be able to have that discrimination domewhat redressed by an averaging out of their property with a less-discriminated-against spouse (provided that their spouse does in fact belong to a less-discriminated-against group). Although this certainly does not alleviate the effects of discrimination entirely, the fact remains that women who are paid a tiny fraction of what their husbands are paid and have frequently sacrificed their careers by switching to part-time jobs or giving up their jobs entirely to provide free childcare and housework for their spouse are guaranteed a certain amount of economic compensation for their labor if they are legally married in a jurisdiction with equitable divorce laws, which they would not receive if they were not legally married. If you go around yelling at them and their spouses about how dare they take advantage of their special privilege to get legally married when same-sex couples can't, you're asking the less economically privileged spouse to give up very significant legal protections, but you're mostly just freeing the more economically privileged spouse of the burden of potential alimony payments. (Okay, in some places you might also be asking a man to give up his legal right to commit marital rape, but since it's not as though even one in ten thousand rapes committed among unmarried cohabiting couples results in a conviction, the fact that it's technically illegal to rape someone you're not married to doesn't really result in much loss of privilege at all in practice. Divorce settlement economic laws are enforced far more often.)
Maybe it just makes queers feel good to be able to accuse heterosexuals of immorality instead of vice versa for a change. Is that it? Maybe queers sense something distinctly dishonest about all the heterosexuals who loudly proclaim, "I'm not homophobic, of course not, I just want it to be absolutely clear to everyone for miles around that I'm quite decidedly 100% straight," and those people's frustrated queer acquaintances have come up with this "How dare you get legally married when my partner and I aren't allowed to?" accusation in an effort to make the "I'm not homophobic, of course not!" heterosexuals finally see themselves in the mirror. Well, I fully support making heterosexuals realize they're homophobic, but that happens to be an utterly stupid basis on which to accuse them, and there are so many much better bases that queers fail to recognize because queers themselves have bought into the same heterosexist myths that the heterosexuals have: the idea that enjoying sex with a member of one's own gender is something that only a minority of people in any culture have ever taken any pleasure in, for example, is thoroughly demonstrably untrue. There are so damned many cultures in which the huge majority of people have had and enjoyed same-gender sex of their own free will, and yet still both the mainstream hetero media and the mainstream queer media continue promoting the ridiculous notion that the majority of humans are born incapable of ever enjoying same-gender sex in the least. Accuse the heterosexuals of heterosexism for believing that myth, and you'll have a definite point - but first you'd have to stop believing it yourself, and it's ever so much easier to just go around compaining that heterosexuals have failed to voluntarily give up marriage when you aren't in the position of being asked to do the same because you haven't been offered the right to marry in the first place. What you have been offered, instead, are numerous privileges like food on your table and a roof over your head, and I'm betting that you're not planning to give all those up anytime soon either.