Answer to second question: It depends on the kid. If the kid has no problem with it and seems to be enjoying, why should I interfere? Its none of my business to put my head between two individuals who seems to be mutually benefiting from an act. If the kid is screaming and crying and appears to be in pain from such an act, I would definitely interfere.
I would be willing to interfere even if there was no sign of distress or pain, if and only if, anyone here can show how the process "that feels good" turns into a mental illness. I showed how it does with verbal abuse and a bit of violence here. [1]
I think it's time for people to back the process they are claiming exists with a simple example. If you can't demonstrate you understand the process, you only believe it. That is faith, not science.