Nestor, you probably misunderstood the guy

He's a scientist and just puts a theory to discussion which is somewhat contrary to the usual viewpoint on the subject and comes with a new explanation.
Prostate cancer is known as highly hormon sensitive, specially to testosteron level, and as such also dependend of your sexual activity. Nevertheless it couldn't be veryfied scientifically if more or less sex has a direct influence on developing the disease or not. Afaik the 'trigger' mechanism is still unknown.
While the attention was so much on the hormon side (one of the usual treatments of prostate cancer is the complete removement of testosteron from the organism, giving you 5-7 extra years), he simply points in another direction and suspects semen fluid to be biochemically aggressive in an unknown way.
His numbers (studies of that size are rather rare) seem to speak for this.
So it is not a statement pro masturbation and contra partner sex, but just the description of an observation
Well, while we're at it: that kind of cancer is a very common one, it's rather harmless if located inside the prostate and in fact it can be there for decades without 'activity'.
But once it grows beyond the prostate and gets into blood circulation it's usually uncurable. Whatever you do, you don't get rid of the cells. The before mentioned hormon therapy can stop the growing of tumor cells for several years in more than 50% of the cases, until those develope a resistance.
There is absolutely NO chance to notice yourself that you're affected by this disease ! Precaution is the only way to go.
Since I don't have current numbers check it yourself via Google if you're interested.
I dedicate this to Frank Zappa, who lost this fight
cheers, Tom