I don't think there's a need to change their mind on whether it is God's ideal
@DustinM. Leave them there, and discuss from that basis - because trying to change THAT is not only very difficult, because it is based on emotion not logic & law so cannot be reasoned away from, but also completely unnecessary.
Because so many situations are not ideal, but still fine and even desirable.
Remarriage of widows is "not ideal", because ideally they would never have become widows in the first place. Pre-fall there could have been no widows, because there was no death. Death is non-ideal, widowhood is non-ideal, and therefore remarriage of widows is non-ideal.
But remarriage is strongly encouraged, even commanded in some cases, and held up in scripture to be a good thing.
So something can be simultaneously not ideal, and also the right thing to do in a certain circumstance.
Many people saying monogamy is ideal will conclude polygamy was only allowed after the fall. So, it was allowed after the fall. God must have allowed it for a reason. What problem(s) of a fallen world does it solve? Do those problems still occur today? Is God's solution to those problems still relevant in those circumstances?
Leave them believing monogamy is God's Ideal. Honestly, you don't know God's mind either - maybe it is! It's irrelevant either way, because absolutely none of us fallible humans can live an ideal life. Don't get sidetracked into having an irrelevant argument you are going to lose and don't need to have anyway. Just move forward into "given you say God has permitted this non-ideal thing to cope with the fall, what does that mean for humans today?".