I agree with what Yumeko-chan has said. Simply put, people go for such games to satisfy their 'dark' desire - Something that everybody has. The liking of different branches of brutality and different genres are due to their personal preferences.
However, it is inaccurate to say that nobody will reenact what they've seen from these H-games. While it is safe to say that most of the people should be mature enough to differentiate reality from fantasy, there will still be that small bunch of mentally immature/unstable people who do not act within the boundaries of reason ( or succumb to their lust ) and commit such offenses such as rape.
Even on these forums, there are immature kids who are downloading content of this nature, which I feel is quite annoying.
Fantasies are harmless. But if you ban the rape games or loli or whatever, people lose their outlet for those fantasies. And THEN you run the risk of some of those becoming real, and I mean that in a very bad way. Like I said, my tastes are different from others, but I can't tell people what to like or not like.
Based on what I said previously, this is only a partial or rather, unlikely truth. People tend to be influenced from a specific source to commit such acts. If you cut that source, then you reduce the likelihood of such an event occurring.
This explains why the ignorant people who put us in the same category as the immature/unstable people mentioned previously have taken such measures to ban these games in certain countries.