Mmmm... brains...
Scientists investigating human sexuality have found that the brains of homosexuals have structural and functional differences from those of “straight” people.
Lesbians appear to have a lower proportion of grey matter in their brains than straight women, giving their brains a more “male-like” structure.
The brains of gay men appear to have structural similarities to those of heterosexual women. They also exhibit the same powerful response as straight women to the sex hormones released in male sweat.

Why this matters? Because people are far more accepting of gays, and civil rights for gays, when they think it's something you're born with. And nobody chooses to have a bigger brain. Well, except those creepy women in the first Star Trek episode with Commander Pike.