I'm ACTUALLY gay and it doesn't even really bother me. I don't know why, I just think I'm the kind of person who doesn't get up in arms about little things like that. But honestly, their inability to think of more clever insults really just attests to their own simpleness in the long run.
Anyway, on topic, I don't think calling people out on it will work, in my experience, there are two kind of people who use words such as this to insult others (mind you this applies to racial slurs as well.):
There are people who are really just un-inventive and unable to think of anything better to say, they probably grew up saying it and it just comes as a second nature at this point, these people will normally shut up about it if I manage to muster enough interest in the situation to say something.
The other kind is a person who is actually a bigot, and genuinely holds baseless hate for homosexuals, and these people- when confronted- usually just get even more belligerent and offensive, because they genuinely hate you (or me) for defending that which they dislike so heavily.
In the end I usually find it's best just to leave it alone, if it REALLY bothered me I'd say something and deal with the fallout, but as is, I just mind my own.