We know that you think yourself clever for having thought up this thing, but, so far, you have been unable to point out its basis in any marriage law.<quoted text>
Just the facts honey.
Marriage is a cross cultural constraint on evolutionary mating behavior.
You have been unable to show there sex is prohibited outside of your "constraint" of mating behavior in maqrriage, and you can't prove that any such constraints within marriage are not the sole discretion of any married couple.
And, since not all sexual behavior (or even most) is doing to do a darn thing to support the survival of the species, I think your reliance on evolutionary mating behaviors as a point of law is rather silly.
Care to try again? Using logic, please.