God never approved of polygamy.
Other than the fact that
- Levirate marriage law makes no allowances or exceptions should the marriage result in polygamy.
- King David was called a man after God’s own heart and he had 8 wives
- King Solomon asked for and was granted wisdom and he had wives in the hundreds
Issac and Rebecka are the Biblical ideal. There was never a polygamous marriage therein that didn’t generate problems.
It is not forbidden by Mosaic Law and there is no punishment for it under the written law. That is tacit approval at the very least considering that common things today (mixing thread types in clothing, worshiping a deity other than Yahweh, a woman not being a virgin on her wedding day) are stoning offenses.