IMO you can call the disparate treatment by gender the result of a “sexist society” but (mostly) not in the manner that kimstu is saying with all this “power” stuff. It’s mostly something else – two other factors come to mind as being more important.
In general, men are assumed to want sex more than women. (IMO this largely true on average, and probably partially biological and partially sociological.) Women are perceived as wanting other things as part of relationships which involve sex. Therefore, in a relationship which is (or is perceived as being) largely about sex and little or nothing else, the man is perceived as having “won” and the woman as having “lost”, since he got his ”no strings attached” sexual relationship. (This is what accounts for the stud/slut disparity, since the man has the aura of a winner and the woman the aura of a loser.)
This is particularly magnified in instances where one party is perceived as taking advantage of another party, specifically in these statutory rape cases. It’s very easy to portray the older man as taking advantage of and using the younger woman for his carnal purposes because that fits in with the stereotype above: he’s getting the sex he wants and she’s not getting a genuine and positive relationship out of it. By contrast, when it’s the older woman and younger man, then the guy is getting the sex, which is a positive for him, and if she’s not getting anything else either, well that’s her choice but no big harm to the guy.
[As I suggested in an earlier post, IMO there actually is more harm to a female victim in this circumstance than to the male victim, this being due to the stud/slut disparity mentioned above. I think the reasons for this disparity are as above but even if you disagree with that, the disparity exists, and societal attitudes of this sort are a strong factor in psychological harm.]
Independent of all the above, there’s a general tendency of people to view – or be willing to view women as helpless victims and men as more responsible for their actions. You see this a lot in cases of crimes committed by groups which include both genders, in which women tend to be successful at portraying themselves as victims and pawns in the hands of controlling men, even when on closer look that turns out to not be the case (or the opposite in some cases).
As a result of the above, when you get to statutory rape situations where the evil and criminal aspect is based on the notion that the older person is leading, controlling and manipulating the younger one, that rings more true to most people when it’s the older man leading, controlling, and manipulating the younger female, as compared to when it’s the notion of the older female leading, controlling, and manipulating the younger male.
Bottom line is that as a result of the two factors mentioned above, cases in which underage males have relationships with older women don’t fit the mold of the victims being damaged by a predator nearly to the same extent as underage females having relationships with older males. (Again, I think it’s actually true that the harm is not the same, but I think the above factors tend to exaggerate that perceived disparity.)