A baby is born. It is quite easy to identify the mother of the baby, but it is not always so easy to identify the father of the baby. That is what marriage is for, to allow society to identify the father of the child and hold him responsible for the care of the child until the child can care for him/her self. Note that this is done for the good of the child, the mother and society in general. The child because nature dictates that it takes both a man and a woman to conceive a child and thus the child fairs best when he/she is raised by both a mother and a father. The mother because it is unjust for a man to participate in the pro-creation of a child and then leave it to the mother to raise the child by herself and society in general since at some point society must deal with the consequences of fatherless/parentless children.
Dan, this is an extremely concise and coherent statement on this whole topic. It shows how even before Christianity these principles were in play and in effect in ALL societies. Anything else other that a traditional marriage is un-natural and will lead to the society's destruction, ultimately, even if it takes some time. But, I think it may not take all that long!