Along the lines of "it's not lying if you believe it," I think "it's not omission if you don't believe it's wrong."
If you know you did something wrong and you intentionally withhold that information from your partner, that's a form of lying, regardless of the motivation (guilt or protection). But you might do something casually which isn't unethical to you but could turn out to be something major to your partner.
Example: putting $20 of your personal pocket money into a slot machine for fun and then walking away. If you have no history with gambling, then you were just having some harmless entertainment. But if your partner was the child of a gambling addict, then it would be a major problem that (a) you did it and (b) you didn't tell them. The partner may feel lied to, because they would be under the impression that you were intentionally trying to hide something, whereas you were just having fun, same as if you'd spent the $20 on mini golf. You wouldn't "confess" playing mini-golf, so why would it occur to you to confess playing the slots?
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker