Is masturbation really a preventative against prostate cancer?

Unclear.: Two studies have suggested a protective association between prostate cancer and frequency of ejaculation (giles et al, 2003; leitzmann et al, 2004). Giles et al reported that the protective effect was seen in men in their 20s, who reported greater than 5 ejaculations per week. These are small studies, and the basis for this association is unknown.

I wish it were: There would be a lot less prostate cancer. Folks who study this have looked at frequency of ejaculation and there's no effect one way or the other -- as I'd expect, since cell division isn't involved.