Born Gay.

The Boston Globe's story on the research into the causes of homosexuality in men makes something clear. "[T]he roots of homosexuality, at least in men, appear to be in place by the time a child is born." Two to four percent of men are born gay. It is probably partially genetic, but as there are gay-straight identical twin pairs, this isn't the whole story. It probably has some component of events that take place in the womb, probably early in a pregancy. Maybe it involves hormone exposure, maybe a virus, maybe something else.

Also, increasing evidence seems to point to male sexual orientation being an either/or proposition. There is little evidence that men are born bisexual, and what sexual orientation is or means in women is less clear:

Researchers at Northwestern University, outside Chicago, are doing this work as a follow-up to their studies of arousal using genital measurement tools. They found that while straight men were aroused by film clips of two women having sex, and gay men were aroused by clips of two men having sex, most of the men who identified themselves as bisexual showed gay arousal patterns. More surprising was just how different the story with women turned out to be. Most women, whether they identified as straight, lesbian, or bisexual, were significantly aroused by straight, gay, and lesbian sex. "I'm not suggesting that most women are bisexual," says Michael Bailey, the psychology professor whose lab conducted the studies. "I'm suggesting that whatever a woman's sexual arousal pattern is, it has little to do with her sexual orientation." That's fundamentally different from men. "In men, arousal is orientation. It's as simple as that. That's how gay men learn they are gay."

An asymmetry between gay men and lesbians flows naturally from a horomone in the womb theory. In the womb, female is the default status until one is exposed to certain hormones that make you male. Gay men could be caused by non-exposure, or reduced exposure, to certain hormones. Lesbians could have biological roots in an entirely different process.

There is more to learn, but the religious model of homosexuality as sin doesn't work. Indeed, it is worth noting that "moral" concern about homosexuality is often strongest when directed at gay men, yet, it is with them that the very early biological basis of the phenomena is most clear (not that science won't learn a lot more about lesbians in due time).