The idea that people can enjoy being raped and/or will fall in love with their rapists as a consequence of the rape. More often than not, the rapist turns out to have acted out of pure love, or winds up loving his/her victim emotionally too. EDIT: vote4love
It depends to me: I definitely love the love hentais. I mean, c'mon, who can hate vanillas, right? Rape hentai is okay to a certain extent. I came across this one Haruhi doujinshi that made me really want to cry where all the girls were put under hyponsis and were raped by various people, animals, and objects. In the end, Kyo's memories were erased before he could save Haruhi and her friends. TOO DEPRESSING. I think it was "Melancholy Princess?" I think that was the worst I've ever read.
I've gotten to a point where I won't even look at a doujin if a girl is smiling in it. >.>
Though femdom rape is good too. Just a matter of preference, I've always been into what I call "diet rape". They don't want it, but they DO. Nothing really brutal where the rape-ee is ACTUALLY going to be scarred for life.