Who the hell would think watching a sex scene would be more damaging than violence? It wasn't that long ago we all shared a one room shack with our parents and siblings. Unless you're talking extreme pornography? That wouldn't be healthy for young developing minds. Most of us heard our parents having sex when we were kids. At the time it might've made us feel sick but I don't think there were lasting effects.
Acts of sexual violence, sex is natural (even if it is uncomfortable to watch in the presence of your children) violence is not, explicit violence is the european censorship concern not sex. An act of love can be empowering but an act of sexual violence is always an act of degredation and it's important that the two are never conflated.
Violence can be explained and you can find it in almost any form of media including news programs. It can also be used as a teaching point about the appropriatness of the violence. Parents still need to discuss the movie with the children after so that they understand context. Domestic violence though would be unacceptable.
Sex is a natural function in most reporductive creatures. Again this is a matter of need for discussion. If the child is brought to understand that it is conesentual sex is natural, the less embarrassed they will be about it and some studies show that leads to lower teen pregnancy and STD rates.
I marked drug use, but anything can be turned into teachable moments. But for me its the worst thing of the three they could see. Course I'm not a big fan of Pharmaceutical commercials on regular tv even.
But that's just my opinion