Even if it were a choice, it wouldn't matter. Even if a completely straight man decides to act against his instincts and have lots of gay sex, it wouldn't matter. That's a perfectly legitimate lifestyle choice and I'm sure someone has done it already. Granted I don't think he could honestly call himself straight if he's able to do that, but female preference sure.
We put sexual instincts on a pedestal for some reason. Who cares what you're programmed to do? We're a sophisticated species. We can ignore it and do whatever the fuck. You do the same thing when you wear clothes, or when you hold your bowel movements until you've reached the nearest restroom. Society is built on overriding instincts.
The argument that some people are born with a homosexual preference is a valid one. That's clearly what happens. However, the push for equal rights should be based on civil liberties and the notion that consenting adults should be able to do whatever they want to. That's a much better argument to stick with, because there are people out there who have a choice (bisexuals), and this protects them as well. This is the difference between being told "well you CAN date the opposite sex, so why not just do that all the time to fit in?" and being respected.