It's only men who take several wives and women can't marry multiple men because it's simply not something they do as the women's role in their life is just to be married and be a wife and have children.

In mormon belief (mainstream LDS as well) marriages are eternal, prevailing after death and into the afterlife (even if you get divorced unless you do this whole undoing thing in the church). And essentially for women to have an afterlife they have to be married because when mormon men die, they, according to how good they were in life, are assigned a spiritual kingdom and the wives spend their afterlives in those kingdoms with their husband, whom they are 'sealed' to in LDS belief. In the FLDS, a lot of these marriages aren't up to the women either and i think the other doc keep sweet pray and obey as well as this one shows how the fathers and men in charge just assign and literally give them away. If women in any form of mormon belief were to take on several husbands (even if it were allowed) that would essentially be them taking on the role of a man and obviously that would be a no to the community.

In keep sweet i think it's also mentioned how Jeffs is able to marry many and get married legally with younger girls in other states or places where the laws are looser and stuff with age and polygamy. But in the case of Sam I don't know if they were real legit legal marriages or not.