Well, not to agree or disagree with the butt popping, the issue is how well your kids respond to boundaries. All kids want them, but they would never admit to it. Life is scary. Especially when you are trying out new wings. Not to have absolute boundaries allows you to fly into all kinds of things, some good, some not so much. Once the limits are set, then everything within those limits is accessible, with no barriers. While there, learning the socially acceptable behavior is a requirement, or those boundaries shrink.
I agree we should never beat our kids. They are our precious assets. But, a swift pop in the seat of the pants to get their attention is sometimes needed. Some kids just are a little harder to get through to. Unfortunately, they aren't born with an instruction manual, and each one is different, so it's really a learning effort on both parent and child. Unfortunately too, is that we often bring parenting skills learned at home when we were growing up, good and bad, and apply to our offspring, until we develop a system that works best for us. We can only hope the more gentle ways work, but that's not always the case.
I really think there should be a test you have to take in order to be a parent. Becoming a mom or dad is easy, but becoming a parent isn't.

At what age is it determined I am old enough to know better?