Because they don't feel the parallel applies to them. After all, they didn't choose to be an ethnic minority, nor could they do anything to change it.

...granted, I didn't choose to be gay and don't think I could convert if I wanted to, but plenty of people think it's that easy. Even if they don't think it's technically a choice, you don't have to live...in a gay way? Ugh, I don't know how to express the concept, that, like, you can just be alone and celibate for your lifetime and not Be Gay, whereas short of Michael Jackson-esque surgery, they're stuck with the skin they're in.

At least, that seems to be the most common opinion I've heard voiced.

Doesn't make it any less stupid, though.