Men who've had sex with more than 20 different women have a 28% lower risk of developing prostate cancer, a new Quebec study says.

They were also found to have a 19% lower risk of aggressive forms of cancer, according to the study from the University of Montreal and INRS-Institut Armand-Frappier.

But the same doesn't hold true for men who sleep with other men - those who've had more than 20 different partners have double the risk of prostate cancer, compared to those who have never slept with a man, the study found.

The study is based on a questionnaire filled out by 3,208 men. Of those men, 1,590 were diagnosed with prostate cancer between September 2005 and August 2009.

Having more female partners may mean a "higher frequency of ejaculations," which can have a "protective effect against prostate cancer," researcher Marie-Elise Parent said in a press release.

Parent said the study's authors can only formulate a "highly speculative" hypothesis that the higher risk in men who have sex with other men may "come from greater exposure to STIs (sexually transmitted infections), or it could be that anal intercourse produces physical trauma to the prostate."

So does this mean public health authorities will soon be recommending men sleep with many women in their lives?

"We're not there yet," Parent said.

More research is necessary, the researchers said.

The study was published in the journal Cancer Epidemiology.