I wonder if this would be immoral to do, what with all the cult of the body and immodesty inherent in fitness today.
I could personally be clothed modestly in public if I were to exercise in public (which I never do), but if you were to own a gym, you wouldn't be able to impose the public to only dress modestly, and coaching people seems problematic too.
I wonder if you could just put these things on the side and not worry about it, since exercising and coaching people are not inherently evil and what anyone does is their own business.