"Marriage" should never have been anything other than a religious intstitution. The Civil Institution was only to protect the parties concerned (spouse, children, etc ....) as a part of a de facto contract - not the protection of the religious institution!
It is my contention that government need only be involved as far as contractual obligations are entered into bewteen parties. I would go farther to state that these contracts, unlike the nebulous agreements of today, need to by concrete and, during thier lifetime, unalterable! IOW - if I make a contract today - the government can not come back in 20 years and change the rules of - who gets my money when I die - what my wife gets if she divorces me, etc ... - how much, if any of my pension she may be entitiled to, etc ....
There is no need for the government to define marriage - instead it must decide who can enter into a contract and whether the act of entering said contract is something the government needs to grant a special status.
Then, whether you will admit it or not, you are advocating for the destruction of the foundations of our civilization.