In the article "Ejaculation: How Often for Good Health?" by Marnia Robinson on Cupid's Poisoned Arrow, Marnia writes:
"Curious, my husband and I began digging around for the answer. We learned that the medical profession considers ejaculation frequency irrelevant as far as prostate cancer rates are concerned. That's right, ejaculation frequency is not a risk factor for the disease. Research studies on the matter have gone both ways. The most recent study we saw found that men who had masturbated very frequently had slightly higher rates of prostate cancer later in life."
She also writes:
"Primates are a funny bunch. Even the sexy bonobos and their cousins the macaque monkeys frequently don't ejaculate when they engage in sexual activity. It seems primates need sex for the social bonds that soothe their brains—rather than mere ejaculation. In fact, comforting contact may be even more vital for pair-bonding brains like ours. In any case, too much sexual stimulation can actually leave people less contented."
In the article, she also writes that a man's body will take care of its own needs via wet dreams. A urologist is quoted as saying whatever a man's rate of wet dreams is, that's how often he should ejaculate.
A search for scholarly articles on Google Scholar has also turned up nothing confirming the myth that ejaculation prevents prostate cancer.
I'm not at all surprised that this hasn't been covered in the news. I can only imagine how many women have been duped into getting their husband or boyfriend off more often, and when she doesn't particularly feel inclined, claiming that it's good for his prostate.
So, if you're worried about keeping your man locked up in a chastity device long-term, or if you're a male who fears for his prostate if you're kept locked up, there's no need to worry.