What is the Link Between Prostate Cancer and Sexual Activity?
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 twenty 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). One day we may eventually discover that there's a virus associated with prostate cancer as well.
Conversely mutually monogamous sex three times a week is reported to be associated with a fifty percent reduced risk of prostate cancer.