Polygamy is a common (and highly ironic given the current state of our culture) objection to the Bible. Why did God allow Solomon, for example, to have so many wives and concubines?
The answer is in the difference between allowing and condoning. God allowed polygamy by not intervening when Solomon did it. However, long before Solomon was born, God laid down the rules for Israel when they set up a king (Deuteronomy 17:14-20). The law, which every king was to keep a copy of and read on a daily basis, specifically forbade amassing horses, wealth, or wives.
That right there should be discussion over. God cannot be blamed for people who disobey Him (James 1:13-14). But we can continue to show that there was a reason God was so strong on this issue. God knew that if the king relied on wealth, or military might, or if he had too many wives to please, then his heart would cease to be loyal to God. Lo and behold, that fear became a reality in Solomon's old age. In 1 Kings 11:4, we see that Solomon's wives eventually did turn his heart away from God, causing him to make sacrifices to abominable idols, such as Molech.
The result of Solomon's rebellion was a war with the Edomites, and a division of his kingdom. God allowed Solomon to disobey, but He did not protect him from the consequences of his disobedience. The lesson we learn from this is that God allows us to disobey Him, but just because He allows it doesn't mean He won't punish us for it. We can choose our sin, but not our consequences.