Anthropology. Prior to patriarchy everyone in the tribe shared nearly everything. Some modern cultures still believe it takes several men to create a healthy child so it stands to reason that ancient ones who shared everything would too. Patriarchy brought about strict control of female sexuality. Why was this needed? Because women were used to mating with more than one man at a time prior to that.
Many anthropologists now believe that collective breeding was more in line with the way people lived pre-patriarchy and that it is a better survival strategy.
Plus, it explains why the female orgasm has such a different timeline than a male one.