The issue is not that the State should stop legally sanctioning marriage. Of course the institution needs civil law to protect the rights of the participants. The issue is the licensing part. There's no reason on God's green Earth to require people to obtain a license (permission) from the State to get married.
Get rid of licensing. That's the business that the State needs to get out of. Look at the mess we have as the proponents work us over in the media trying to propogandize one or the other position here. We get blithering idiots (read that "experts")on one side saying that gay couples can't properly raise children because they are, well, gay. This is followed by equally blithering idiots who say that gays make better parents because they are...sensitive people. No proof one way or the other, have you, but people buy into the crap.
In reality I don't know which position has validity but I suspect neither. In any case they're talking about married couples (gay and straight) raising children and we don't require a license to have children from anyone. There's no requirement for a heterosexual unmarried couple to ask the State's permission to have children. And they do have them - lot's of them. (Refer to my request in the previous post.)
We can argue the morality, ethics, religion, etc. angles until we are blue in the face but it won't justify a State licensing requirement. We already have immoral, unethical and irreverent people who are "married with license." The license requirement doesn't assure good behavior or even serve a necessary function of the State - other than yet another tax to pay so the State can hire more government employees to process the unnecessary licenses and SEIU can garner more dues from government union membership which is collected by yet other government employees in the treasurer's office to be delivered to the unon bank account. (It's late and I'm now officially rambling.)