It should be obvious to anyone who watches the news that, at least in the US, when it comes to how the public at large handles depictions of gratuitous violence or gratuitous sex, reactions usually vary wildly. We're used to violence on movies and television shows, to say nothing of the perpetual sport we as gamers partake in through gunning down criminals and generic "bad guys" through the course of any FPS or action game. We're "desensitized" to this, at least when our impetus is killing enemies in the name of a greater good, and most people don't really balk unless said killing is exceptionally graphic and excessive.
Yet when it comes to sex, and more specifically sexual crimes like rape, we typically have a threshold of tolerance significantly lower than what we're willing to put up with in terms of violence. Watch a movie about killing enemy soldiers and most people are okay with it, but if that same movie involved explicit depictions of rape (as is historically an unfortunate part of warfare in the past) people would have visceral responses of disgust.
Bitmob has a very thought provoking article up about this subject, and Omar Yusuf ventures forward with his own ruminations on why we as a society will tolerate participating (at least virtually) in acts of murder for entertainment, but are usually filled with repulsion if a game asked us to engage in rape. It mentions, as these topics usually do, the Japanese eroge game RapeLay, which has been knocking around the internet a lot lately.
Upcoming PS3 title Heavy Rain features a scene in which the player's female protagonist is forced by a male character to perform a striptease, and Quantic Dream's David Cage has said that the experience is not meant to be sexy but is instead intended to make players feel upset and uncomfortable. By reversing gender roles and making a largely heterosexual male demographic perform a degrading, humiliating act instead of simply watching it, Cage hopes that it will provoke a mature discussion on sexuality and sex in general. Whether his intention comes true or not won't be discovered until the game is released, but the scene in particular is already courting controversy in advance of the title's launch.
As Yusuf says, and I agree, criticisms against games like RapeLay are well-placed, and I think that it's a good thing that most of us find ourselves disgusted by them and would be incredibly uncomfortable if a game expected us to sexually assault enemies instead of killing them. But the overall message is that we, as a society, don't approach depictions of violence with the same measured tones as we do the subject of sexual crimes. I am certainly guilty of blasting away enemies in most action games without ever really thinking about what I'm doing, and this is largely a failure of most games to give you a reason to reflect to begin with. Some have tried, but the experience often falls short of socking you in the gut with the realization of what, in fact, you're actually doing. We accept violence and murder in our media, but are disgusted with sex-based crimes– the latter is a good thing, because such violations should repulse and not excite us, but I do have to wonder why the former is just accepted "as is" without any of us really bothering to think about it.