Well I did do that. I got a rabbi to marry my wife and myself. We were married. No one stopped it or objected.
So there isn't an issue of "choice" here at all. No one is restricting anyone else's choice.
You chose to get married, and you did. They can choose to get married, but their choice won't be legal, or provide them any of the legal benefits you enjoy. Why? Why do you feel you deserve better than they do.
Because my marriage resulted in children, has lasted now 22 years (as of Sunday) and represents stability for society. In other words, an ideal outcome for the state. While other marriages may have less than ideal outcomes, no one goes into a marriage wanting a less than idea outcome.
But now you've moved from "choice" to "legal recognition." Moving the goalposts. Sure sign of losing.