Excuse me? There is an obvious lack a historical / sociological perspective behind such opinions.
Marriage is a legal institution that protects the family, specifically the children. It's meant to provide a stable environment for their growth and development. Although not really enforced by the community now, marriage used to be an unbreakable social bond, for the simple reason that you can't divorce your children.
It's not a right for grown ups, but for the little ones.