Just to throw in the conservative Christian (my) view... Marriage exists as a covenant before God and is a reflection of the Christ-Church relationship. It is a picture of two becoming one...so that there is no division between the two anymore. All of Christian history is pointing forward to the Wedding Feast of the Lamb, when God's people are united with their Lord as Bride & Groom. This really has nothing to do with a "big Man with a big stick up there."
I do have to agree with you that outside of the sacred context, I see no overt reason for marriage itself in terms of purpose. There are, however, all sorts of statistics that point to the success of the arrangement in terms of the well-being of children. That is not to say that children of divorce or single-parent homes cannot succeed, but there is great advantage in the arrangement of having a committed, influential man and woman in the life of children. It provides balance and stability that simply cannot be replicated. Again, that being said, so much rests on the quality of parenting of father and mother, not the mere presence of both father and mother in the home.
Not even within the conservative Christian vantage point is there a no-way out mandate to marry, however. In fact, the New Testament encourages singleness in certain regards...just in a very different manner than our society looks upon bachelorhood.
I hope this is helpful to somebody.