Is it normal to let child watch violent scenes but not sex scenes?

I realise you may be talking mainly about films but I think the same thing applies. I'm not saying lets all let our kids watch some people having sex and I also don't agree with people who say violent films and games cause violence, what I am saying however is that if as you said think your kid is too young to know about sex or understand it how is he/she going to understand the complexeties, reasons and right and wrong of violence