I agree fully with Pandoren. I think him pleasuring himself in other people's houses is a little out of line...I personally wouldn't allow him to continue working in my home if I were to catch him doing that, and I would bring it to his superiors. He's running a risk there, I think.

How young are the girls? I don't think that's unusual unless their age of consent is questionable. Regardless, if his use of that bothers you, I do think you have a right as his partner to let him know it bothers you.