God never intended polygamy. "For this reason, a man will leave his father and mother and be united to his wife, and they will become one flesh" (Genesis 2:24). Polygamy was never condoned by God, even though it was practiced in the Old Testament, as were many acts that were repulsive to God. The polygamous marriages that you see in the OT were examples of jealousy and strife, for us not to follow in those footsteps (Rachel, Leah, and Jacob). Just because God chose to not immediately destroy those who perverted His Word doesn't mean that he approved of their behavior. If you change the definition of marriage, you open up a whole new bag of worms. Gay marriage, polygamy, beastiality...well, some might argue that it's not unacceptable to love a horse.
In fact, it wasn't David's 7 wives that ticked JHVH off. That was fine and dandy. It wasn't until David covetted another mans wife that it was considered a "sin".
Moses had more than one wife. Solomon. Ezra. Esau. The list goes on.