The answer should be no, there is no valid reason to forgive your spouse for cheating on you. But, in reality at least half of people do forgive. That is if you believe estimates that indicate 80% of married persons have cheated on their spouse yet only 50% have ended in divorce.
If it’s in your nature to forgive, than I might consider the following to be a valid reason: it only happened once, it was a huge mistake and the cheater admits it. He or she is truly sorry for it and can manage to convince you it will never happen again.