It is really all about gays being able to take advantages of all the benefits of married life without having to adhere to the conventional standards of marriage. After all, that’s what being gay is all about: flouting conventional standards.
I say this because it has been my experience, with the literally dozens of homosexuals that I personally know, that the vast majority of gays simply have a variety of axes to grind with society. They feel that they have been rejected by the greater community somewhere along the line, and in response, they reject the greater community's tenets, and if gays can annoy, pester, thumb their noses or poke you in the eye while they do it, the more they like it.
I’m not here to say whether gays should be allowed to marry or not. Frankly, I think the question is a pointless one for a very obvious reason; the purpose of marriage, strictly speaking, is to erect (oops, did I just say erect in a post about homosexuals?) a series of psychological, legal and religious barriers to wandering penises and vaginas. Ill explain, and try to do so without bringing all sorts of emotional gobbledegook into the equation.