Let's initially leave the "cheating" on the side and focus on the "don't tell..."
A good friend once said that if her husband were cheating on her, she definitely would not want him to tell her. Because as soon as he did so, it would become her responsibility. Whereas him, he could then say "my wife knows it".

Interestingly with your statement "... if you don't tell your spouse" you exactly emphasize this.
You imply that it is (or at least might be) cheating, but as soon as you tell your spouse it is not cheating anymore.