It's true that while sexuality isn't a choice, whether or not to have sex is. People can choose to go celibate, or go against their own sexuality-- but those can be very unhealthy choices.
The thing is, OP, I guarantee that pretty much every time you've heard somebody claim that "homosexuality is a choice", they're talking about the sexuality itself. They're not saying that gay people should just have sex with women; they're saying they should just become straight (which is, of course, an impossibility).
Slight tangent: If you really can choose who you're attracted to, OP, I envy you-- I'm attracted to both sexes, and I can't even choose which sex I'm attracted to at any given time.