Lets refer to the beginning of the Bible, which is Genesis with Adam and Eve. If your friend is supposing that man was not designed to be with one woman, then ask him why did God not give Adam more than one wife in the beginning? In Gen 2:24 it states "Therefore a man shall leave his father and mother and be joined to his wife, and they shall become one flesh." , not wives. The more than one wife scenario was started by a man named Lamech (Gen 4:19), who was from Cain's line, and others followed suit with concubines, etc.
Just b/c people did have more than one wife, doesn't mean that God condoned it. If you examine situations where people had more than one wife, it was always extra drama involved that could have easily been avoided. Jesus addressed this issue with the divorce question asked by the Pharisees in Mark 4 by pointing to the beginning where marriage is the union of one man and one woman, and that Moses instituted divorce because of the hardness of hearts, not God. Paul backs this up by stating that each man have his own wife, and each wife have her own husband in 1 Cor 7:2.