I don't think viewing homosexuality as a genetic mistake makes you a homophobe, you neither fear nor dislike gay people. On the other hand it is awfully rude.
As an example, I view blue eyes as a genetic mistake. I'll say that here because this boards tolerance for rudeness is a lot higher than most places, and I think people know it doesn't mean i think less of any people out there with blue eyes. But saying that to someone, blue or brown eyed, in real life, is a pretty rude thing to say.
Whether being gay is a genetic mistake or not is the sort of thing that I think could make an interesting debate, but only with someone I am comfortable enough with that i know feelings won't get high. I personally think it is genetically advantageous. (not to the gay individual, who isn't reproducing, but to their genetic line as it causes them to serve as extra support for their heterosexual relatives. Also to their species, as high populations densities tend to correlate with homosexuality, which leads to less reproduction, which is good at high population densities.)