Is marriage a necessary social institution?

Traditionally, marriage is the socially preferred institution for sexual activity and child raising in many human societies. Most of us probably know examples that stand outside such tradition. Probably all of us know of examples that follow tradition. When you ponder these examples does the necessity of marriage become evident to you? Is there something about society in the USA and globally that evidences the necessity of marriage?

Here I am not asking if marriage is a right. I am not wondering about the distinction between religious and civil unions. I am wondering about the efficacy and need for having the social institution of marriage.

I am tempted to say more and offer my opinion, but I am not really interested in hearing people affirm or argue against my thoughts. Besides, I am not fully certain what I think.

I want to know what others are thinking and why they are thinking it.