My daughter has been in ballet since she was 3 or 4 years old (She's 9 now). There have always been 2-3 boys that have participated in classes (For one, they're a hit with the ladies that severely outnumber them). They are incredibly athletic once they hit their mid-teens and I wouldn't be surprised if they were in some kind of weight lifting/ strength conditioning class.
If you're worried about ballet being too feminine, you should look no further than Lynn Swann and Herschel Walker. Both incredible football players. Both of them took ballet.
Swann:
Transcript: Lynn Swann 02/17/99
Walker:
Walker Balances Bulk With Ballet - NYTimes.com