Your sons no longer need parents. They are adults and can manage on their own. Therefore your current marriage is a "mutually sterile, pointlessly opposite gendered half of marriage".<quoted text>
You don't make sense.
I have two sons.
And since you believe that marriage is "essentially a cross cultural evolutionary constraint on mating behavior", you no longer need to be married.
Your wife's body is no longer going to produce offspring. She no longer needs you to provide for your offspring. There is no need for you to continue to hang around and enjoy the rights and protections of marriage.
According to your own definitions, you should move on to someone else and raise another family--that is if you are still potent.
Otherwise, dissolve your marriage. You can still live together and seek out the same legal protections through attorneys that you claim same-sex couples can access.