Lot is considered to be a righteous man (he was saved from Sodom after all). The angels pulled him back to save him, not to prevent him sacrificing his daughters. His actions were not directly condemned are therefore implicitly accepted.
And this is only one example. The Bible (the OT in particular but not exclusively) is Not A Very Nice Book as far as its attitude to women is concerned.
I agree with you that a marriage composed of one man and many women is not a good thing, particularly if that style of marriage is cuturally embedded.
...logically if you divorse your wife and marry another that's considered adultry in the new testament. So if you marry another while still married then that's certainly adultry.
It doesn't follow. Jesus was speaking in the context of current Jewish culture, which folowed the monogamy model. He explicitly stated that divorce alone is bad, not remarriage alone. Extending this to a polygamy model, divorce is still wrong.
Notice also that the entire discussion on adultery and divorce in Matthew 5 is completely centred on the male viewpoint. Women don't get a look-in. There also appear to be discrepancies between Matthew 5, Matthew 19, and Mark 10.
Matthew 5: "anyone who divorces his wife, except for marital unfaithfulness, causes her to become an adulteress, and anyone who marries the divorced woman commits adultery."
Notice the "except for unfaithfulness" clause, and that it's the
woman who is the sinner.
Matthew 19: "I tell you that anyone who divorces his wife, except for marital unfaithfulness, and marries another woman commits adultery."
Now both the man and woman are sinners. The unfaithfulness clause remains.
Mark 10:"Anyone who divorces his wife and marries another woman commits adultery against her. And if she divorces her husband and marries another man, she commits adultery."
And now the unfaithfulness clause disappears. Curious!