I'm curious... do you believe the same thing about people who lust for sex with farm animals? If so, are all sexual lusts other than a lust for the opposite sex part of a "genetic abnormality" that people are born with? If not how do you explain the differentiation between what people lust for and their genetics? Where is the cutoff point?
Yeah, that's where the whole arguement falls apart. Are people born with leather-fetish or rubber-fetish or fill in the blank fetish? There are all sorts of odd things that turns on certain people, and the only real explaination for those is it is a learned behavior. Sexuality is a powerful positive reinforcement mechnism, and to discount its ability to shape ones sexuality is just plain ignorant.
I would also throw in the pedophiles. Are they born with a lust for little girls and boys? Is there a gene we can isolate for that? It all falls under the same heading. I do not think it is natural in any of these cases, but of course that is just my opiniion.