The problem is (with these polygamous groups) is that they raise girls to think that this is normal, acceptable, and something to vie for.  These are generally not women who are otherwise  well informed and making a decision that reflects a fulesome consideration.  They are basically brainwashed into thinking they want this life.  This is why in wider society you don't tend to see 16-18-20 year old young women married to 60 year old men (or vice versa), and polygamy/affairs are generally  not considered  appropriate.b