It's neither natural nor is it a gift from heaven. It's an entirely human-made social relationship that clearly is not for everyone. It has been created, thrived and survived because it has been adaptable in it's form to recognize things like joint property rights, the recognition of women as persons before the law, polygamy, same sex relationships (is that husband and wife really? You decide for yourself) and that there are circumstances where an orderly dissolution of the relationship is best for all parties involved.
Oh and there's something about love and getting along with one another that has some part in the whole equation too.
