It's not a law, and if it is it's dumb and no one follows it.

That said, MANY parents choose to put a clause in their divorce agreement saying that they will give an opportunity to the other parent to meet a new significant other BEFORE the children meet them. But this is usually a mutual respect thing and something done when EVERYONE'S focus is on what is best for the kids involved. This does not work and would be dangerous in abusive or unstable situations, hence the reason I'm sure it can't possibly be a law.