Some gays and lesbians oppose marriage because it would force them into a situation that is traditionally heterosexual. To marry and/or have a family would sort of be admitting that straight people have something that should be copied.
Some gays and lesbians support marriage for the same reason. It's a basic human right to have a lifelong partner and maybe a child. It shouldn't matter what gender the participants are.
I think I support gay marriage. I don't agree with the lifestyle, but it's not my place to tell two consenting adults what they can or can't do in their bedroom. If two people fall in love, they should be able to marry, period. Well, almost. Marriage is one of those holdovers in our legal system that came from religion. There ought to be some secular form of marriage, like Denmark's registered partnerships. That takes religion out of the question of gay marriages.