Not only is marriage a set of intimate and personal promises, it is a legal entity. The state has a stake in marriage and family. The selection of one's mate, the rearing of one's children, the protection of and distribution of personal property, and the decision to end a marriage are NOT strictly personal issues. To legitimize children is key function.
Marriage makes institutional provisions for a preferred method of producing and nurturing children.
Legitimating sexual activity and living together is a Latent function of marriage. Marriage as a Permanent Commitment Marriage has always been viewed as long term, more or less permanent. At the turn of the century, when the divorce rate was very low, individuals usually married later in life, when the financial means to fund a family was established. People were older and more established. Now, as we marry earlier each decade, individuals are increasingly less prepared for marriage - both emotionally and financially.