Your cheater does not get to decide for you…
…whether their cheating is a big deal (or deal breaker) for you.
That is YOURS to decide.
Now, I understand many a cheater will TRY to decide this for you. They will tell you all sort of lies about how the cheating is merely a symptom. Some will even directly blame you for it.
Still, YOU get to decide!
That power is yours. You do not have to surrender it to the cheater. In fact, I would encourage you not to do so.
Cheating IS a big deal!
People who do not have an invested interest in downplaying cheating understand this. You are not crazy to thinks so, too.