I don't really agree I think it's very rare that two people have full sex in the main dorm, and when they do it's under the covers, no one from what I've seen has openly shagged in front of people.
They can masturbate, simply they have to do it in the loo. I genuinely don't get the issue with that rule at all.
I can understand thinking all sex or sexual activity should also be banned, but not that as safe sex with no nudity is permitted, then wanking in the common areas should be allowed to.