Well okay this is why: He's not the type to get pissed off easy and he's stuck in a situation where the characters who do harass him are all female so he's not going to punch them since he's not an asshole and it tends to always be multiple people when in those situations so of course that's ridiculously hard to deal with plus there's no male friend in this.
I'm not asking for a reason why he acts this way. He's a pushover, that's why. I'm stating that I think the decision to make him a pushover was a terrible mistake, and is probably one of the reasons leading to the decline in Japan's birth rate.
Your justification, however, is so wrong on so many levels. Why are you implying that punching them is the only decent response? I'd never punch anyone who wasn't physically threatening me. Additionally, despite all the language I'm using in my explanation here, it's entirely possible to say each and every one of these things without cussing the person in question out.
The decent responses are simple. Tell the lady who makes dirty jokes all the damn time and literally pulls your pants down in the middle of class that if she keeps slandering your reputation, then you don't need her in your life.
As a matter of fact, as a general rule, if anyone tries to sweep you along in the current of their random whims without seeming to care at all about your actual opinion on the matter; tell them to stop it or get out of your life.
As for one of the other ones she's a sadist that's the point any guy's meant to get pushed around
Also, this line of thinking is not the normal reaction. This line of thinking is the reaction held by masochists. Not that there's anything wrong with being a masochist, but if you aren't a masochist then you stop allowing the sadist to have her way. You don't just sit there and let her push you around. If you do, you may in fact by a masochist good sir.