The number of sexual partners you have may increase your risk of prostate cancer because it increases your potential exposure to infections. The number of ejaculations you have, however, does not. Prostate cancer is at the stage where cervical cancer was some 20 years ago. We knew then that cervical cancer was correlated with the number of sexual partners a women had, and now we know it is caused by a virus (and can be reduced substantially by a vaccine). We may eventually discover that there's a virus associated with prostate cancer as well. Conversely (and to boost survival of the species, we think), mutually monogamous sex three times a week is reported to be associated with a 50 percent reduced risk of prostate cancer.