Because we live in a dominance-hierarchy culture that envisions masculinity as power-over, might makes right, a real man takes what he wants, etc. Non-patriarchal cultures, where dominating others is not seen as desirable, don't have this problem.
Also, there’s this — for the same reasons, a culture of domination:
For example, the CDC’s nationally representative data revealed that over one year, men and women were equally likely to experience nonconsensual sex, and most male victims reported female perpetrators. Over their lifetime, 79 percent of men who were “made to penetrate” someone else (a form of rape, in the view of most researchers) reported female perpetrators. Likewise, most men who experienced sexual coercion and unwanted sexual contact had female perpetrators.
Among those who were raped or sexually assaulted by a woman, 58 percent of male victims and 41 percent of female victims reported that the incident involved a violent attack, meaning the female perpetrator hit, knocked down or otherwise attacked the victim, many of whom reported injuries.
