And what will happen to them if they just see tits? What really? Will they get any shock or psychological trauma? I don't see here anything special. And as far as i know, porn isn't allowed on TV, only erotica, so "holes" there won't be shown. Anyway the best way to protect them from porn is a turning off internet, when you're out of home.
You can't stop such movies, although you can make your kids understand to stay away from such things.
If you straightly just say them not to watch a such films, their interest to this will be only doubled.
This is not the main problem, better watch your kids not to smoke something or drink in their childish years, that's the main problem you have to take care of.
Educate your kids to be good, friendly and smart (in order to them not to do anything, what's out the law), some day they anyway will find out, where they were born from. xD