What about the issues in the Old Testament with families like masters sleeping with their servants and men having many wives or even the issue of "inter-family" relationships and the like? Was it a population issue? When did the law change? Why was it okay then and not now?
Great question! The problem is, the Bible rarely make commentary on historical events. If it did, we would see notations like "[and this was not only sinful but STUPID because God's plan for marriage is one man, one woman for life, and bad things happen when we disobey His commandment]."
The fact that sinful, unwise behaviors are recorded (without commentary) in the Bible doesn't mean it was OK any more than newspapers reporting on crime means they condone it. They're both just telling you what happened.
Hope this helps!