Much has been made about the connection between black and gay civil rights, and the tendency for some parts of the former to deny any such connection, to the disappointment of progressives. David Weigel discussed the divide yesterday in this piece for Slate.
The principle difference argued by anti-gay blacks is that homosexuals chose to be homosexual, whereas no one can choose to be black. If you find yourself arguing with these people and try to directly counter the assertion, you're making a mistake, for the nature/nurture debate only skims the surface of a much deeper issue, which is that of personal liberty. It doesn't matter if homosexuality is chosen or if it is inborn, it is a personal liberty that no one has any valid reason to deny.
Getting to the heart of the matter is important. It makes the picture clearer for everyone, and makes it clear you know exactly what you're talking about. (The opposite example is a creationist who attacks every possible detail of evolutionary theory without a connecting fiber in any of his arguments.) For this particular case, you'll then be able to skip past sticky discussions of race and other traits you are born with and talk about why homosexuality is or isn't a personal liberty. As I've said before, there is no logical basis for their religiously or culturally motivated bigotry, and they will lose that argument every time.