Even if it's supposed to be harmless when it comes to a kid, it's giving them early conditioning to reduce responsibility for their actions. It doesn't even have to apply to rape, although it does have that connotation. People use it when they're being little shits running around screaming and annoying people, too. Tell your kid to fucking behave instead of being "oh he came into the house I just cleaned, covered in mud, teehee boys will be boys", while expecting little girls to be the pinnacle of politeness.