>that his heart turns not away

The idea there is to not multiply wives to a degree that it turns you away as it did Solomon, the dude has like 700 and it specifically mentioned they turned him away. He put more attention and care into them than God. If we were to apply the same logic you are using for that verse then having money is bad as well.

Is polygyny is prohibited then you gotta figure out what it means when it says

Exodus 21:10 KJV
If he take him another wife ; her food, her raiment, and her duty of marriage, shall he not diminish.