There isn’t a darn thing illegal about schtupping the staff when they are willing and consenting adults. Certainly no reason to fire someone. Indicative of who he is as a person, sure, but no one should think he’s going anywhere because of it. At least not in this lifetime.
There certainly could be some EEOC implications if promotions and raises were involved. Staff members that didn't get boinked might have an axe to grind, etc.