Whenever an affair has been revealed, it often puts the relationship in serious danger. Would you want to stay with your partner if they were cheating on you? The partner who has cheated is clearly not happy with something in the relationship, and has to decide if they really want it to continue.
The person who was cheated on has to decide whether they think their partner can commit to them again, or if they will ever trust them again. Often they can’t, and the relationship breaks down.