And that is why Luther made that call. Later, he regretted it and said that in effect, he screwed up and caused Phillip to remain in sin. The OT did not forbid multiple wives, though in later years (mainly around the time of the exile in Babylon) it fell out favor. By the time of the Maccabees revolt, the leaders of the time had outlawed polygamy.
Funny story. Someone asked Luther shortly afterwords if he would take another wife. He said something along the lines of "Katy would kill her! I have enough trouble with one wife!". Luther loved his wife very much (you can see this in his letters), but had no desire to add another one.
In the same lines, I have often thought the David and Solomon would not have gotten themselves into trouble had they just had one wife.
The worst part about having 300 wives and 600 concubines?
Having 900 mothers-in-law!