The reporters are sensationalizing this report; the Professor fuelled that a little. The reason stated, that "semen is a very potent and strong brew of lots of chemicals which, because of their biological reactivity, could be carcinogenic if left to lie around" doesn't exactly means that it's masturbation that does is, but rather, frequent ejaculation. The suggestion that masturbation equals ejaculation, no matter how frequent, is strong, perhaps too strong. While it may be a common thing, masturbation is not the only frequent way to get rid of that stuff.