Researchers at the University of Montreal and INRS-Institut Armand-Frappier found that men who had slept with more than 20 women during their lifetime were 28 per cent less likely to develop the disease.
They were also 19 per cent less likely to develop an aggressive type of cancer, compared to those who had had only one female sexual partner.
However, the same did not apply to gay men, according to the Canadian scientists. They found that having more than 20 male partners doubled the risk of prostate cancer and made an aggressive cancer five times more likely. Sleeping with one male partner did not affect the risk.
Meanwhile, men who were virgins were almost twice as likely to be diagnosed with prostate cancer as those who were sexually experienced.
The findings are from the Prostate Cancer & Environment Study in which 3,208 men answered questions about their lifestyle and sex lives.
Of these men, 1,590 were diagnosed with prostate cancer between September 2005 and August 2009, while 1,618 men were part of the control group.
Overall, men with prostate cancer were twice as likely as others to have a relative with cancer, but the study also found a possible link with their number of sexual partners.
Lead researcher Professor Marie-Elise Parent, from the University of Montreal, said: “It is possible that having many female sexual partners results in a higher frequency of ejaculations, whose protective effect against prostate cancer has been previously observed in cohort studies.”
According to one theory, large numbers of ejaculations may reduce the concentration of cancer-causing substances in prostatic fluid, a constituent of semen.
They may also lead to fewer crystal-like structures in the prostate that have been associated with prostate cancer.
Suggesting why the same did not apply to male partners Professor Parent admitted she could only provide “highly speculative” explanations.
One explanation she said “could be that anal intercourse produces a physical trauma to the prostate”.
The age at which men first had sexual intercourse, and the number of times they had been infected by a sexually transmitted disease, had no bearing on prostate cancer risk.
A total of 12 per cent of the group reported having had at least one sexually transmitted infection (STI) in their lifetime.
Professor Parent added: “We were fortunate to have participants from Montreal who were comfortable talking about their sexuality, no matter what sexual experiences they have had, and this openness would probably not have been the same 20 or 30 years ago.
“Indeed, thanks to them, we now know that the number and type of partners must be taken into account to better understand the causes of prostate cancer.”
On the question of whether promiscuity might now be recommended in health advice to men, she said: “We’re not there yet.”
The research is published in the journal Cancer Epidemiology
By The Independent.co.uk.
Additional reporting by Press Association