For me, gay marriage is on a level of fundamental human rights and equality. That everyone should have the same rights under the law is literally one of the central beliefs that the United States was founded on. If we're giving benefits to couples who are married, gay couples should be entitled to those same benefits. [1] I don't see how anyone can say anything to the contrary. If I heard something remotely convincing then I would probably accept it, but I haven't. Largely the opposition of gay marriage is just bigotry because it makes someone uncomfortable. [2] The issue here is that those beliefs aren't legitimate, they just deny another human being rights that they should have.

[1] I'm not sure whether we should give benefits to married couples at all, but if we are then all couples should be eligible for them if they desire.

[2] Besides the opinion of a single person on HN, I've never seen anyone against gay marriage because they are against marriage as a whole. Its always that it offends them in some way.