It's wrong to marry more than 2 women.
Brigham is a married man twice over and is courting yet another woman.
Brigham wants to be with the third woman.
Brigham does some self reflection and ends his first two marriages so that he can be monogamous with the third woman.
Brigham marries the third woman bringing his total number of wives to three.
Brigham lives a normal life going forward having just one wife.
Brigham's local community decides they cannot continue to put up with him marrying so many woman and exiles him.