Decriminalizing merely removes the statutes that make the actions illegal from the books. Legalizing provides a positive action by the legislature that says this is now legal. For example, in Colorado pot sales were made explicitly legal, not merely no longer illegal.

While it might seem a technicality, it should be decriminalized. This removes it as a crime without giving it state approval.

Of course, underage would still be rape, regardless of whether consensual or not.