I can only think of a couple places:
1) Gospel(s): Jesus says women at the well had many husbands and current one is not her husband
2) Paul says something about an elder being the husband of only one wife
I don't think the first relates to polygamy. The second is what I'm talking about with "mild rebuke." Not an outright condemnation, but clearly a statement that monogamy is the ideal.
Slavery seems to be affirmed in the NT.
Not affirmed at all. Tolerated, but the NT suggests it is not ideal. "Man-stealing" is condemned, and masters must view Christian slaves as brothers. Doesn't make sense to enslave your brother. And since you're commanded to love even your enemies, it doesn't make much sense to enslave anyone.
So Abraham, Jacob etc were just "tolerated" ?
I don't recall Abraham having more than one wife at a time. He had another wife (Keturah, or something?), but I think she was after Sarah died.
You can't read about Jacob without understanding that he was a very flawed individual. His name means something like cheater, or liar. His polygamy was a result of his being cheated into marrying the wrong sister, and was hardly presented as something to be emulated.