I think that more attention being paid to - and more sentencing and less social acceptability for, even in nasty subcultures - male-on-female rape, the more will fall onto other kinds of rape, just as a side-effect. I think that the reason rape by women isn't seen as a big crime is the general perception of female powerlessness in re sex, and the reasons man-on-man rape isn't much noticed is because it happens to "men who are like women", ie men who are fucked. So it's all tied into societal perceptions of masculine and feminine sexuality, regardless of the actual sexes of people involved, and those perceptions are fucked up.