A one night stand is no different than a long term affair. There are lies, there is the actual infidelity, there is broken promises and loss of trust, there is the loss of faithfulness in a relationship. A ONS is just as much cause for leaving as any other infidelity, and just as forgivable as any other infidelity.
My H had 3 ONS's with 3 different women, and a 4th woman he had 4 "ONS's" with over an 18 month period (like, no dating, no relationship, just a booty call every few months). In my eyes, I'm disgusted that my H could treat another human being like that, use them up and throw them away like trash. That's NOT the man I married. But I was able to forgive him for that. And I know others here have forgiven their spouses for the same type of thing, and many have forgiven their spouses for different things.