I think it's a gap between 16 and 30+, really.
Our adult classes are pretty evenly filled between men and women. We have only 55 min workout time, due to the full schedule of the school, but there is a little room to socialize and form friendships. I think that's a girly thing, guys seem to have less time once grown and with both feet in life and job.
In out area a female dominated class would not go well, because we have such a warped sense of femininity around here. (though there are many times when there are only the women present...our poor instructor!

)
It's the boyfriend thing, usually...need more geek and nerd girls....