Lets just say I would let my kids play manhunt, if they can't get very far then will either lose interest or be determined and get past it.
As for exposing them to graphic content, I have been playing 18+ games since they first came out and it hasn't had any negative effects on me. I might be desensitized to death and violence but is that a bad thing?
Never had any nightmares or become some social deviant. Most kids probably wouldn't like to play resident evil (you know, the older scary ones) after the novelty of playing an 18 wore off.
As for sex, I know when I was a kid, I just wanted action so I skipped all the cut scenes. If you take a game like dragon age, it took me about 60 hours to sleep with one of the NPC's (I didn't know you could), I doubt a kid is going to be giving gifts and dragging out the story just to sleep with a NPC or give them gifts (they would probably sell them for cash. I doubt they would realize what they were seeing anyway.
On to swearing, they are going to learn it somewhere and at some point no matter what you do (bar keeping them locked up under the stairs). Just have to follow it up with teaching the were and when to use it, don't want little Timmy telling his teacher to "fuck off".
So to top it off, nothing is too far. I think we put too much cotton wool round our kids these days, toughen them up a little.