Here is where you not only go stupid, you also show why a gay can never comprehend, let alone dictate, anything about marriage.<quoted text>
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".
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.
Marriage is a life-long commitment. While childbearing years come and go, the relationships birthed continue, and the circle of life continues. We anticipate grandchildren and even great-grandchildren. We hold our role as not just a model of marriage and parenting, but grandparents.
A mutually sterile ss couple can never experience those relationships. The roots of a family, branching out and literally blending with others.
This is a perfect example of why homosexuals need the self-identity and confidence to establish their own relationships. They don't fit marriage, no matter which way you go.