I agree that parents need to be more diligent in what they let their kids watch in terms of TV shows and internet content. However that is a different situation then a parent consciously taking a kid to a rated R movie, as the parent is actively involved in that situation.
I do believe its a question of morality, as everyone parents their kids differently based on their personal beliefs. Its not up to you or anyone else to tell me what to do just because its against your personal beliefs. Such as the example that I shouldn't take my kid to an R rated movie, that should be my choice as a parent even if you don't agree because its against your personal beliefs. For example some people's religion doesn't allow them to watch R rated movies at all, even as an adult. And so I don't like when people try to make laws for moral reasons just because people do things that go against their personal beliefs, that I think is wrong. We as a country, the US, should not legislate in terms of morality. Unfortunately this has changed over the last couple decades, and I vote against it every chance I get.