Research showed that Lotharios with a lot of notches on their belts are significantly less likely to develop the disease.
Compared with men who have had only one sexual partner during their lifetime, those with a score of more than 20 conquests have a 28% reduced risk of being diagnosed.
But the same is not true for gay encounters, according to the Canadian scientists. In fact, having more than 20 male partners doubled the risk of prostate cancer.
The findings are from the Prostate Cancer & Environment Study in which 3,208 men answered questions about their lifestyle and sex lives.
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.”
The study found that men who were virgins were almost twice as likely to be diagnosed with prostate cancer as those who were sexually experienced.
Compared with those who had only had one partner, men who had slept with more than 20 women were 28% less at risk of prostate cancer of all types.
They were also 19% less likely to develop an aggressive type of cancer.
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.
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% of the group reported having had at least one sexually transmitted infection (STI) in their lifetime.
While having one male partner did not affect cancer risk, having more than 20 doubled the chances of a diagnosis. It increased the risk of a less aggressive cancer type five-fold.
“It could come from greater exposure to STIs, or it could be that anal intercourse produces a physical trauma to the prostate,” said Prof Parent, who admitted the explanations were “highly speculative”.
The research is published in the journal Cancer Epidemiology.
Prof 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.”