Sorry but no. You are missing the point entirely. You are promoting a certain behavior and rewarding those that engage in that behavior with financial goodies. That is a recipe for disaster. This allows the government to social engineer.
The state can offer a marriage contract that allows a man and woman to be married and no legal problem arises from such. It is when you reward that behavior with government goodies is when the problem starts.
Sorry but no doesn't cut it. How does marriage law treat anybody any differently than anybody else when anybody can avail themselves of marriage if they meet the requirements of any states law?
Whether tax law treats married couples differently from single people is a separate issue for marriage law. Do you think tax incentives are unconstitutional or just a bad idea? Because I'd agree it's a bad idea but they certainly are not unconstitutional.