Everyone deserves a second chance in love. A cheater who cheats once should get a second chance. After that, forget. Two cheats should be it. The phrase "fool me once shame on you, feel me twice shame on me" is appropriate in this case. A second cheating instance should be it for the couple because at that point, it becomes a pattern.
We live in a world that is full of temptation and opportunity. We are all imperfect. If a person is a fundamentally good person then I don't it is wise to turn your back on them for exhibiting ordinary human weakness. Trusting someone who betrayed you is not a thing easily done, but the alternative may be unhappiness.
Trust is something that is earned by one's actions and conduct. It is can be lost by the same actions and conduct. Just because someone has been unfaithful once does not mean they will be unfaithful again. They can be trusted if you feel they have again earned your trust.
I do not think that somebody who has been unfaithful can be trusted to remain faithful afterward. Once somebody is unfaithful in a relationship, it is difficult for that person to be trusted again. Such an act ruins and the relationship beyond repair and just places way too much burden on the couple.