Marriage is necessary. Even if you exclude the spiritual, legal, and intra-relationship aspects (which I don't believe you can), it is a universal commitment made to the world (via proxy of family and friends) that signals you have chosen another person to be your life partner. The necessary aspects for society are to have a single method of doing this.
If not, you will have "I'm Married" vs "I'm bonded" vs "I'm partnered" vs "I'm dating" etc... each can have infinite meanings with no single social construct.
Today, if someone say's they are married, you know the basic social construct and meaning. (whether it is upheld is another matter).
Now, go back and include spiritual, legal, and intra-relationship aspects, and you have a wonderful rich and intrinsically flexible construct to make your own. How these are defined are up to you. That is the real beauty of marriage.