No two people have a "right" to marry. Every marriage has to be approved by the society in which the two are being married. This is old, settled law although the issue is mostly ignored in our modern era. You have to post the banns of marriage in advance so people can object if they have a reason. There's a last chance at almost every ceremony. Members of the community must witness the marriage.
It's not just two people deciding how they want to share their lives. It's a community accepting and agreeing to support a new family in conjunction with the new couple.
We can re-define it so it's not a community thing, but that's what marriage has historically been.
“Members of the community must witness the marriage.”
This is an important principle that libertarians just don’t get. A marriage must be witnessed, or else it is not a marriage. Why? So that it is public knowledge whom you are married to and why.