The nuance is that “sex” requires consent, because sex without consent is rape. So sex with a non-consenting person (including all children) can’t exist, because it’s actually raping children.

It’s more of a nitpicky difference than the others because I’d bet your average adult doesn’t bother thinking about that nuance, but it does carry a different subconscious weight. One is considered fun, happy, and almost everyone does it; the other is a monstrous act that has no justification.