Is ual identity a biological essence or a social construct?

I am very interested to know; what makes gay men gay? Or rather, what factors undermine this either decision or realization? I've gotten to know (but I don't know them well enough to ask them this directly) some great people who happen to be gay, and I have been wondering, what is the gay philosophy? There are not that many books on the subject and it's not as publicly discussed as other social arrangements, so I am just very curious. Not being prejudice or anything.