Gay people can only marry in 3 states, New York, Massachusets, and now California I think. It should be everywhere in the United States.
Secondly, no, marriage is not just for children, and there is no reason why gay couples can't or don't raise children. They do. Sometimes, they are impregnated through invitro (sp?), like Melissa Etheridge, or sometimes a woman is impregnated the old fashioned way, or a gay couple adopts, or whatnot.
Thirdly, marriage comes with many benefits, such as special tax status, visiting rights, etc.
As for the research you asked for, here is a rundown of Kinsey's scale that he first created in 1948:
http://members.tde.com/ben/kleingrid.html
Here is a recent article discussing some of the theories of the biological basis for homosexuality:
http://comp9.psych.cornell.edu/dbem/...rrelations.htm
Though there is no definite identification of a "gay gene," or any such thing, that does not mean that homosexuality isn't at least in part biological. However, what it emphatically is NOT is a choice.
And I ask you again: when was it that you made the choice not to be gay? What factors went into your decision? Have you regretted it? What does the thought of engaging in gay sex make you feel? If you wanted to, could you just become gay and go have sex with a man on a whim?
I don't really want your answers to these very personal questions. I'm just pointing out that the "choice" argument is inane.