I recently had dealings with a neighbor who was so convinced of the value of recycling that she sought community action to force people to recycle.
To my mind, if the benefits of recycling are so weak that social coercion must be used to create behavior that would otherwise not occur, then it becomes obvious that the recycling itself is of very little value.
Do you picture heterosexual marriage the same way? Is the benefit of encouraging and recognizing lasting monogamous, heterosexual unions so marginal that state sanction must be used or "marriage" will cease to have the importance that we might wish it to have?
If the state had never meddled in the institution of marriage, would we have the divorce laws that we now have? My parents were Catholic and it simply didn't matter that the state permitted divorce. The Church did not permit it.
Well obviously the concept of marriage, family and heterosexuality is weak enough that we now have scores of people running around saying that homosexuality is normal and should be treated as equal to heterosexuality and that marriage should be however anyone wants it to be defined.
Also we do not have the right to association in this country. Discrimination law suits are common. By saying that the state has no right in the marriage business will still not stop the leftwing from trying to make it illegal (by penalty of lawsuit) to not recognize homosexuality as being equal to heterosexuality. They will simply continue to bring lawsuits against anyone who does not agree with them. So no matter what the state will still be involved in this issue. We will simply have agreed to let them bully us into having no representation on the issue of marriage, family and sexuality in open society.