One recent theory to explain this is based not on genetics per se but on epigenetics, in which heritable phenotypical changes can occur without an actual alteration of the gene sequence. Unfortunately for the theory, though:
…so far the theory “is not supported by any data.”
Indeed, Andrea Ciani, an evolutionary psychologist at the University of Padova, thinks that a variety of factors, including genes and epigenetics, influence sexual orientation. “It’s a little bit vain to think we’ll find the answer to homosexuality as a whole.”
Which is just as well, because I really didn’t want to get into those particular biological weeds. And to the degree that it’s genetic, it’s unlikely that it’s a single gene, because it doesn’t seem to be a very evolutionarily useful one. It’s more likely some complex of genes, each of which is useful on its own, but when combined, results in the person being gay.
Anyway, I have a more parsimonious explanation. Whether from gene or womb, most people (like me) are born straight, a few are born who wouldn’t do it with the opposite sex on a bet, and some (perhaps a lot) are born in between and really do have a choice. That is, they are bisexual. My guess would be that it’s a skewed distribution toward heterosexuality, with a long thin tail of homosexuals, but a big bulge on the heterosexual side of the scale of folks who can go either way. That is, they are to one degree or another bisexual.
My theory would explain why some of the most vociferous opponents of homosexuality often (more often than one might have guessed) turn out to be attracted to the same sex — they have a choice, and they feel morally superior to those upon whom they project their own bisexual orientation, and thus assume that people who don’t uphold their own standards of morality are merely weak-willed. These would also be the people who really could be counseled to go straight for religious reasons — they really had been influenced by their postbirth environment, and were capable of going the other way.
So this might explain the twin conundrum as well. The twins who are both homosexual either were born homosexual or were born bi and both chose homosexuality. The ones where only one twin had that trait (as with the Collins brothers) were born bi, and made different choices. I know that if I were heterosexual with an identical twin, I would find it mind blowing to be told he was gay, because then I would be wondering why I wasn’t. But in Jason’s brother’s case, maybe he’s thinking: “Well, I decided to do the marriage-to-a-woman-and-have-kids thing, but I can see his point of view.”
In any event, the mystery continues.