This happened to me once (well, it's happened more than once but I'm just going to relate this one story!) ... I think I was about 15 in high school. There was a group of "popular" kids who apparently had decided they wanted to embarrass me during the lunch break. So 2 girls and a guy come up to where I was sitting along this brick wall outside and they're laughing and the one girl (imagine a total "valley girl" accent) says, "So like, are you like, a girl or a boy?" (more laughing). So I respond, "Why are you asking? Do you wanna f__k me?"
That pretty much shut them up because they definitely weren't expecting that - then while they were still trying to formulate what I said I added, "Because you know, I can't really figure out another reason why someone would want to know what's in my pants." The lead bimbo huffed and then turned and the other two followed.
They tried messing with me a couple other times after that and I just shut them down every time. I think they got bored after that LOL