- It's pointless / meaningless - you're not going to change the heterodominance by not getting married in solidarity (and you can change the institution from the inside - no really! - where the implication seemed to be, although I may be a tad sensitive, that only married people can talk about marriage seriously and change society's views on it)
- Gay couples do not need to be patronised by you straight couples
- I want to / I enjoy it / I love my partner / I loved my partner who needed immigration status with me / financially it made sense (which is particularly difficult, I think - this arguments often ignores the fact that it's a privilege gay people can't enjoy - loads of gay couples "want" this, and it's not a valid reason until you acknowledge that it didn't affect your decision to get married, but then, if it's how you stay together/what you've always wanted, who am I to tell you not to do it? It's not up to me to order your priorities).
- The country club metaphor (i.e. I don't want to join a club that excludes gay people) doesn't work for marriage because it's so integral to modern life
- It's a copout by couples who don't actually want to get married and who are using gay discrimination as an excuse (see second bullet above)
I think it's possible to have a more nuanced position than these, though. I do have ambivalent feelings about marriage, and the prohibition on gay marriage fits in with those feelings - gender prescription and roles, etc., which I have vaguely mentioned here. But it's not as if when gay marriage becomes legal in NY I'm running straight to City Hall, dragging TOH behind me to finally give in. It's just that it's yet another reason why I am unsure about whether marriage is right for me. And that ambivalence would be less likely if we had more nuanced, less prescriptive roles for "husbands" and "wives," which, I believe, would occur with legalisation of gay marriage. So for me, it's not the country club metaphor, but the army metaphor. I would not join the armed forces if they prevented gay people from openly serving; on the other hand, I have very ambivalent feelings about the armed forces (although I do see the point of a professionalised military and have no desire to remove it completely), and am unlikely to sign up, ever, anyway - but that is in part because of a macho culture that rewards things that I do not want to reward, in the way, I think, that marriage can reinforce gender roles and stereotypes. So it's a mutually reinforcing problem, and not quite as clear cut as the abstainers and anti-abstainers make out.
I also have to say that I have a lot of misgivings about weddings and their gender presentation in general. So this article I found amusing, if unsubtle...
UPDATE: Interesting follow up about going to the "inside" and enjoying the privileges...