Don't let senior level students audit junior level classes? Why are they even there besides the reasons outlined in the problem? This is a problem with the University because they don't want to acknowledge a problem. Do they really think that's going to last? If it was my daughter who left the class because she was harassed I'd be very angry.
Why do people see this as even remotely unethical? It seems to me that it's great that plenty of younger people are getting essentially free tutoring sessions and fun friendships/relationships out of it. Unless there's sexual assault/battery being alleged here, this is both ethical and good for all the students involved!
Maybe it would help to imagine it's an all men's school and it's young men who feel uncomfortable being hit on while they need to learn. I'm not saying it's any different, I'm just saying shifting it this way be help you think about how it's a power imbalance.