As Ernest eloquently explained, the primary reason for faithful monogamy was economic, and that is assuming a child is involved. If people were promiscuous, then many children as well as their mothers could have easily ended up without support because no one would be sure of who the fathers are. Actually, I said economics was the primary
non-religious reason for the immorality of cohabitation, not that it was the primary reason. I said it that way because different people place different weight on religious reasons and I was trying to make my argument as broadly applicable as possible. Also it wasn't just the economic security of the children, but the woman that was protected by an insistence upon marriage instead of mere cohabitation, even if no children at all were involved.
Again, that is my view. But the bottom line is morals can and should be rationalized. It makes very little sense to follow them if there's no reason to.
On the other hand, we humans sometimes have insufficient data to understand the rationale of God, so just because we can't comprehend the reasoning is by itself an insufficient reason to reject the moral teachings of one's religion. I'm also going to have to disagree with you Scott in your belief that cohabitation is not immoral according to the scriptures. However, like all sins, cohabitation can be corrected, in this case either by marrying or by ceasing the sin.