It is sane for the government to set aside an institution specifically for those who have procreative potential.
If the government realizes these folks are the source of the next, last, or future generations, then they are simply recognizing biological reality. I can see concessions and benefits to such a group.
Let’s call them the MARRIED group.
The whole problem is marriage in not a institution of the STATE. The STATE did not institute marriage. I merely recognizes it. It pre exsisted the STATE. Now that the STATE is involved the STATE wants to say what MARRIAGE is and is not. This is something that the STATE does not have the power to do. They can pass laws all day long saying gays can marry. But they can NOT actually do so. They can only get the STATE to call their contract marriage but marriage it will never be.