I think if the Sensei owns the school then he can do what he wants (within reason of course). But if the school is owned by someone else then the sensei must take some responsiblity for his actions. I don't think this is restricted to dating. This can also be applied to teachers doing business with or borrowing money etc from students.
For most of us, aikido is a social activity. Its an opportunity for many to meet new people and make friends.