What do you have to say to the 40-year-old man who says he has a natural right to have sex with 10-year-olds and for the state to punish him for it is just as wrong as state support for slavery was?

It seems that the only effect this "natural right" term has on sufficiently curious interlocutors (who will not fall for your rhetorical trick) is to signal that you are more stubborn than people who do not use the term.

I would tell the man that he should just take a class or read the Wikipedia on what a natural right is if he's so interested in it and using it to protect himself. I would tell him that commenting on it in this manner embarrasses himself and exposes that he has no education on the topic.

Any involuntary contract imposed on any individual is axiomatically immoral and unethical, as I've said below. There's no need to use such extreme examples.