it's become a cliche in hentai, but this is a rather interesting subject...
if you think about it, outside of hentai, rape fantasies aren't terribly prevalent at all. In the old days, rape was used to make people pity the rapee, hate the rapist, and that was about it. The idea that rape is somehow a good form of sex has grown out of more and more violent sexual practices.
Bush says "there's no sex in violence" but even if you read ancient texts, it's clear that violence, or some form of rough physical interaction, has always been used as a way to turn people on.
Even "romance" novels written by women FOR WOMEN will often contain a sex scene like the following:
He swept me up in his powerful arms, and before I could even catch a surprised breath, he'd whisked me to the bedroom, ripped off my blouse, and thrown me onto the linen sheets.
As he removed his shirt, I came to my senses, realizing all too well what was about to happen. I sat up on the bed and tried to tell him that this was impossible, that we shouldn't do this, but he simply pushed me back down onto the soft sheets and captured my mouth in a hungry kiss, silencing my futile protests.
Now, seriously, this shit is old school. But it's almost just as much rape as in hentai. The girl didn't initiate the sex. She tried to stop it even. But the man's physical strength and perserverance ensures he winds up balls deep in her by the next paragraph.
And don't tell me, "That's not rape! She consented after he kissed her!"
Does it matter WHEN the other party consents, so long as they do?
In most hentai, as been said, the girl usually starts to enjoy the rape at some point. But does it stop becoming rape after she gets that glossy-look in her eyes in the last panel and says, "Please rape me more" ? Does it stop being rape when she starts moving her hips on her own, even if she's verbally protesting? Is the example I used above rape up until he kisses her, and magically not rape after?
Rape in hentai is, more often than not, the ultimate form of rough sex. (okay, guro is, but...). Sure, they sometimes use drugs, tentacle goo, magic and the like to keep the girl a 'mindless' slave. But it's just often a plot element trying to disguise the standard "caught up in the heat of the lustful moment" that is in plenty of romance novels.
so if there is "sex in violence" then to understand why rape is so prevalent in hentai, we need to understand what kind of violence is sexy. And, as they say, sexiness is in the eye of the beholder. Many fantasize about being dominated by their lover. The variable is the level of dominance that turns that person on! A nice little housewife might fantasize about being "taken" by the pool boy, and a sadist about being "raped" by unknown assailants, but they both want the same thing, rough sex with some kind of imbalance in control.
Control...
Control.
Controlling. Dominating.
Dominance. Violence.
Capacity for violence. Strength.
Strength. Ability to control.
Ability to control. Another's inability to control.
Inability to control. Weakness.
Weakness. Helplessness (lack of capacity for violence).
Helplessness. Submission.
Submitting. Allowing dominance.
Being dominating. Lack of control.
Control.
and rape, as you've seen, is all about control. It's control of sexual situations.
And, before I go, let me say I'm in no way advocating rape. I'm not. I'm just trying to explain why there is so much violence in sex. It is, quite simply, because violence is sexy, at least to the more primative part of our brain that we still have from when humans were trying just to survive in the wilderness. You can throw every single social, progressive, egalitarian concept you want up as reasons why rape isn't sexy, but, unfortunately, those concepts can't gain complete control over the purely physical, primitive attraction to violence.
Guns and dicks have a lot in common for a reason.
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. Uwasa no Midori-kun just posses me off to the fullest. 
