What's next? Incestuous marriage? Hey if it's two consenting adults why not? Then if two can consent, why not 3 or 4? Hell as long as there is consent why stop there?
Society is being destroyed by all of this crap.
I promise you that the next thing that will become acceptable will be sex with children. Homosexuality is already becoming more and more acceptable and it is just a matter of time until children are involved.
Legal child prostitution. If a minor can legally have sex with an adult at twelve, but still requires a guardian of some kind, then will this guardian decide if he/she can have sex? How much will the 'guardian' charge for this?