1. The difference between 17 and 18 is highschool and college.

2. Most parents are probably not aware of the sex scene in the game. And probably only about 5% of parents are actually complaining about this kind of stuff anyway.

3. Video games are made for kids. And you should not put porno action in a kids game.

4. Even if the kids have scene worse you should still not intentionally put porno action in something they will play.