This may sound conceited, but I think that I could have sex with 80% of the men (and/or women) who see me dance. I'm not a professional dancer, by any means, but I know how to shake my ass. And I've picked up people/been hit on after someone has seen me do my thing, when they did not seem interested before.
I think an underlying issue is that there's something about a body in motion that both draws attention away from potential flaws, and draws attention TO what your body is capable of. This is part of why runners are so sexy--it's one activity we see (not just in a gym) all the time! They aren't just decorative, they're focused and determined and sweating and into it. Which is...very very very sexy. To me, anyway. Which is basically what counts. :D
They also tend to fit the slender ideal, which is kind of problematic, but on the other hand, seeing the men's running club go by shirtless is pretty awesome.
And both of these activities exude confidence; both in ability and general "I may be making a fool of myself but it's what *I* want to be doing". Which is tremendously attractive.