I'm curious - do we know what the biological basis for gender-specific attraction might be? It seems to me that gender is a learned distinction. We aren't born with the ability to distinguish males from females, in contrast, for instance, to the ability to distinguish between foods that smell edible verses those that smell spoiled. Indeed, young children often seem to have very peculiar ideas about the nature of sexual difference.
The preponderance of heterosexuality across different societies, and especially across different species, suggests that there is some underlying biological predisposition. To play the devils advocate though, perhaps it's significance has been overstated. I'm more attracted to tall thin women than short fat women, which is certainly cultural, just as I salivate at the smell of beef and recoil at the thought of eating dog. Perhaps the fact I'm more sexually attracted to girls than boys is just as learned.
Memes like 'men shall not lie with men' have historically made sense from a public health point of view - in just the same way memes for monogamy did. There are obvious epidemiological reasons to minimize the non-essential exchange of bodily fluids, whichever way they're flowing. I see an analogy to injunctions against pork. I'm certain the incidence of homosexuality would be dramatically higher than ~10% in a society where there wasn't such a stigma attached, Ancient Greece being the canonical example.