There seems to be a lot of confusion over whether or not abstinence (orgasm denial) causes prostate cancer in men. The reason for this confusion is due to the fact that there has never been any conclusive data from any medical study on the subject.
Oh, there have been many studies done. Too many to list here, but the problem is simply a lack of empirical data either way. The one thing the entire medical community can agree on, concerning prostate cancer, is that we know more about its treatment than we do about its cause.
One of the main problems with any study of this nature rests with the honesty of its participants. Also, running a “control” group is NOT easy. After all, who is going to tell a bunch of grown men that are not permitted to ejaculate for the next twelve months (or longer) while another group will have to ejaculate several times a week? Who is going to keep the abstinent group honest?
What we do know is that the older you get, the better chance you have of developing prostate cancer. Your race may (or may not) also have an effect on that possibility. Ask any prostate researcher and he/she can tell you more things that don't cause prostate cancer than any that might cause it.
So, if you are worried about putting your guy into long-term chastity, you can always learn to milk his prostate or become very good at ruining his orgasms. Either of these methods would be an effective prostate cancer preventative, if it is, indeed, caused by abstinence.