I'm not so sure it's fair to say that male-on-male is overlooked -- not by the victims or their advocates at any rate; just look at the endless Catholic pedo-scandals, which are primarily man-on-boy.

What is maybe not taken as seriously, arguably, is woman on man / woman on boy sexual assault, not least because the attitude of many men is they (think they) wish they had been thusly "victimized" by comely Miss Johnson in the 10th grade. It fits with idle heterosexual male sexual fantasies. However it can be just as devastating.