Over any given 12 month period, it's rare for there to be more then a 10% difference between the two. There are still more male rapists due to higher rates of male-on-male rape compared to female-on-female rape, but neither stat is very high in absolute terms. Contrary to what you've probably heard in the past, most men are not raped by other men. ~80% are raped by women. And more men in total are raped every year than women, making sexual assault more of a men's issue than a women's issue.