I'd reply, "You should ask Mikhail Baryshnikov what he thinks of that theory".
This. I love this. I can't get over how far apart the reality is from this particular stereotype. The bozos may call them twinkle toes, but (male) dancers are usually seen as athletic and sexy.
I also can't believe people still throw around the insulting assumption that all male "you knows" are effeminate. They can be/do anything, just like everyone else. There are "you know" football players, martial artists, etc. Pick any activity and I'm sure you can find a "you know" who is interested in it.
If you really feel the need to engage these morons about their assumptions about ballet or the people who perform it, maybe you could look at them wide-eyed and say "Gosh, I thought dancing was for coordination, athleticism, and creativity. How does it make you 'you know'?"
Treat them like they should give you a scientific explanation for their idiocy. Make them take you through the actual supposed 'conversion' process.
If they can actually do the above without sounding like and idiot, and realizing they sound like and idiot, and continue to insist that ballet is bad/will make your son 'YK', just shut them down with "I love and support my son. Period."