I wouldn't call a ONS 'something stupid' selina. 'Something stupid' is your spouse forgetting your birthday, saying your bum looks fat, or pouting because you did something they didn't like. That's stupid. But a ONS is breaking a sacred promise, whether you are actually married or are in a committed partnership.
You can forgive if you want to, of course. But don't belittle the seriousness of what happened.