Fake in some way! Even if there were a bit of truth to it, how would they even determine something like this? Are the house keepers cleaning up semen in the showers all the time?
Semen is mostly water anyway. Plus, it loses its viscosity after a few minutes. I can't imagine that affecting the plumbing systems. Hair causes more trouble than anything else.
The sad thing is with most dorms and rooming situations, the only privacy you can really get is in the shower until you roommate is gone for a while. So I guess I can see how a lot of guys would wank in the shower. Fortunately I always had a enough privacy in my room to rub one out frequently.