Hmm..

Um many people have sex with their parents as adults but they keep it in the dark because of people like you. If you're worried about them being underage, after all, that has nothing to do with him being their parent/guardian.

That's called incest, and is considered a punishable crime in almost every country. It's forbidden in many religions and scientifically its wrong. Animals (including humans) are constructed to not be attracted to their bloodline; genetically its terrible because it leads to birth defects and many health problems.