Imho, the bisexuality wouldn’t be the issue. The fact that one partner was not aware of the sexual orientation of the married, on paper lifelong partner, is. Bisexuality, in contrast to pansexuality, has a focus on the, well … sex of the partner. The bisexual duality can not be satisfied by the binarity of one partner, leaving a lot of space for unhappiness, uncertainty and confusion, in a state where certainly might have been exspected.