"Whether I sleep with them or not, they still leave."
Supposedly a true quote by a sensei although I got it secondhand.
If the two folks are consenting adults, then they are consenting adults and that's pretty much the end of the discussion. If the sensei wants to risk dojo harmony then they pretty much get to do that, assuming they own the dojo. If you don't like it, vote with your feet and go somewhere else, which, is a suggestion I would endorse. If the sensei is finding all his/her dates in the dojo it's probably not a healthy place.