I too am glad we've been able to keep this going without it being locked. Lots of interesting points of view.
As far as the born gay argument; I don't think it's as straight forward as that. Some people are most definitely born that way and know from a very early age. My Ex's brother for example. He knew from a very early age.
I've known women that have been straight for their entire lives, one really messed up relationship put them off men, and they decide they'd be happier with a woman.
I don't have any experience to talk about transsexuals, so I'll use Jim Norton as an example; straight guy who's into transsexuals.
Straight men serving Life in prison will engage in homosexual relationships while inside and then go home to their Wife and Family. 'gay for the stay' as they call it in America.
Like Alison said, Things that happen to you in life can put you off the opposite sex. Rape, abuse etc.
I think when it comes to sexuality you can't put a definitive label on everyone. Some people, sure.
“Life is like riding a bicycle. To keep your balance, you must keep moving.” - Albert Einstein
“Life is what happens to you while you're busy making other plans” - John Lennon