I'd feel more comfortable showing a 12 year old a violent movie than I would an adult movie.

Or maybe put another way, if my child was at a neighbors house and one of the parents watched an adult movie with my child I would have a huge issue. If they watched Terminator or something similar, I would have much less of an issue.

They're not even close to the same thing.

I think your analogy is a bit off. The "watched it with" really changes the dynamic. There are quite a few activities that I'd take issue with adult neighbors doing with someone else's child. That's an entirely different question from a child doing things on his own or with other children.

Not defending the ratings system here, but there is a big gap between M and R/X or whatever your country uses.

A better question would be if you would be more comfortable with your child being shown porn or snuff movies. For me the answer would be neither, in about an equal measure

They're different things, yep.

I think parent comment is asking you to consider why you think young children watching violent movies is way less of a problem. E.g. "Terminator or similar" - why draw the line there?