Since the vast majority of sexual assaults occur among acquaintances, and since the vast majority of sexual assaults involve alcohol, it stands to reason that men and boys are sexually assaulted at rates far, far in excess of those actually reported. It is fair to say that underreporting of female-on-male rape is at epidemic levels. Raising awareness of this crime ought to be a primary focus of sexual assault counselors.
The reasons for this underreporting are obvious. Society does not accept that men can ever be victims of sexual assault. Even if it happens, it is not deemed to be a something negative since the victim is often physically aroused. The evolutionary reason for the double standard in thinking that sexual assault is only bad when it happens to women is simple: only women can get pregnant.
Here's a good article about it: http://kdvr.com/2013/10/10/expert-female-on-male-rape-an-increasing-problem/