biology != genetics
There are several avenues available to make a legitimate case that sexual orientation is biologically based without ever mentioning genes or, shudder, selective advantage. Furthermore, it's possible to mention genetics without having to bandy about the gay gene concept.
Talk about prenatal hormone exposure. Talk about the psychosomatic effect on a pregnant woman of perceived overpopulation. Talk about sex-swapping Amazonian frogs. Talk at length about the sexual habis of Bonobos.
And then get back to the original point: There's a bit of a contradiction in declaring being attracted to someone of the same sex is a choice, while also asserting being attracted to someone of a differing sex is not a choice. Besides a couple of fairly confused people I knew in highschool that thought it was trendy, I have never known of anyone that claimed to be able to choose to be attracted to someone... much less choose to be attracted to someone that's 180ยบ from what they "should" be attracted to.
Consider: "Not My Type." Why does that phrase have any meaning? Furthermore, assuming you have one, why should "your type" be whatever it happens to be?
There are ideas about hip:waist ratios, pentagonal/geodesic symmetry of bone structure, and other supposed advertisements of fertility. BUT, what about the guys that like slim-hipped girls with small breasts? Or the girls that like slim-hipped girls with small breasts, for that matter?