@Sleepii See, there is the problem. You are ignoring the part that it's not often actually portrayed as rape in this.
Second, you are ignoring the point where we put ourselves in the males place. Seeing out own position, so I myself imagine myself in the male's shoes.
It's why I and many others aren't as bothered by it when it's the older lady with the young boy. Because when we were young boys, that was our dream. That is a position WE WANTED TO BE IN back when we were kids. As a little kid, me and my school friends would talk about how hot our teacher was and stuff like that.
If a young boy is able to bang an older female, then he's a champion in the eyes of many. In society for thousands of years, it was a mark of pride.
There are countless more social implications to the issue than it simply being adult and child. You are over simplifying it.
And the fact of the matter is, these manga are written to fit our fantasies... and these are geared towards men. As kids, we all wanted a sexy onee-chan.... and so this manga fulfills that fantasy....
Just think about that.
This manga is one made specifically to fill the fantasy that people had back when they were kids.
On the other hand, females fear rape much more than males. That's just an objective fact.