A Question on Homosexuality

Has there ever been a pair of twins where one of them is homosexual and the other heterosexual? I ask because I’ve never really heard of one twin being gay and the other being straight.

If homosexuality is genetic then that would mean not one pair of twins, in all of the world, will differentiate on sexual preference. But if twins do separate on sexuality then that, of course, would mean homosexuality is not genetic; given that twins obviously share the same genetic makeup

Now, I’m certainly not implying being gay is a choice, I’m just wondering how one becomes attracted to the same sex instead of the opposite sex. Maybe someone can help me out on this: Have you ever heard of one twin being gay and the other being straight? And, either way, what does it mean?