It is quite possible that "an adulterous people" was a reference to the Crow's polygamous marital arrangements. This 1892 report to the Secretary of Interior makes several references to the fact that among many other plains groups, the Crow were still frequently practicing polygamy. The author notes that polygamy was still very common on the Crow reservation, but also notes its practice among the Gros Ventre, Blackfeet and some other Northern Plains Tribes. I don't have a Crow ethnography handy, and can't get to the library today (got baseball tickets), so this is the best I can do.

Can you give more info on the context in which the term is used in the movie ? I ask because 'adultery' has historically been used to refer to things other than sexual-infidelity.

For example in English translations of the Old Testament it's used metaphorically to refer to the tribe of Israel turning to other religions and so being unfaithful to the Jewish God.

Elsewhere it's used to refer to political or economic corruption.