What Absol said. I'll add that it shouldn't matter, though. Gay sex is no better or worse than straight sex and gay relationships are no better than straight relationships. It's only because of homophobes that it matters, in much the same way that homosexuality is a risk factor for suicide only because of homophobes.
Originally Posted by noparlpf
More science stuff:
Sexuality probably isn't set by birth. There's a concordance of 50% in twin studies, meaning that 50% of gay men with identical twins have a gay twin. This is much higher than chance or fraternal twins, but it's less than the 100% you'd expect if genes + womb environment determined sexuality. There's something else going on, and it could be genetic (there are some ways identical twins aren't perfectly identical genetically), it could be that the womb environment isn't exactly the same for both twins somehow, or it could be that something in a child's environment after birth but before puberty affects sexuality as well. Don't ask me what, though.