It comes from the notion that games are for kids under the age of 10, and having explicit sex will keep parents from buying their game when they read it on the back. Yes, the game is rated for adults, but they want to sell as many as possible. For example, I was 11 years old and playing FFVII when it came out, and it was all like "god damn" and shit like that, and I ate that shit up.
Why violence is acceptable? I think parents realize just how bad it feels to commit acts of violence for normal people, and how good it feels to commit acts of sex for normal people, and feel children are more likely to go out and knock fuck (this was a typo, but I like it! knock fuck!) after watching it more than go out and stab somebody.
To tell you the truth, this shit is stupid, cause I was watching hard hard hardcore porn by the time I was 10. Really, the internet has us all spilling our seeds anyway. Just, seeing firsthand how knocking a girl up ruins your life if you are young, I wouldn't want my kids fucking either. Cause, it will ruin your life.
Also, Germany, the fact sex is taboo, makes it all the more fun.