Men who had slept with more than 20 women lowered their risk of developing cancer by almost one third, and were 19 per cent less likely to develop the most aggressive form.
In contrast, men who slept with 20 men doubled their risk of developing prostate cancer compared with men who have never had sex with another man.
Researchers at the University of Montreal believe that intercourse protects men, and men who are more promiscuous have more sex than those in monogamous relationships.
However, for homosexual men the benefit is lost because of the increased risk of picking up a sexually transmitted disease, and the damage to their bodies from intercourse. However gay men with just one partner are at no greater risk.