Unfortunately, I forgot to include an important point in my last post which dealt with precisely this issue. Here it is (belatedly):
It doesn't matter whether homosexuality is genetic or not. Homosexual acts are capital offenses regardless. The Canaanites were Canaanites genetically (they certainly didn't pick their ancestors) yet the Torah ordered their extermination. Ditto with the unfinished job with `Amaleq. And as for the Kutim ("Samaritans"), they were excommunicated, forbidden to ever become Jews at any time in the future, and even banned from the World to Come by 'Anshei HaKenesset HaGedolah (I mean, is that brutal or what?). All this being so, I think that qal vachomer the "genetic predisposition" to homosexuality (or to theft, murder, or what have you) is just tough nuggies. And I say that not out of cruelty but just out of simple factuality.