We feel that a lie by omission is still a lie. No matter how small. Now, does that mean that we tell each other EVERY LITTLE DETAIL of our day? Yes, pretty much we do. If it's significant, like dealing with sex, or some sort of romantic thing, then we tell the other person every little detail. Now, there are certain details we have chosen to allow omissions on. One of these subjects is kissing. We choose not to want to know about when the other person is kissing someone else. It is a don't ask, don't tell policy. We don't care about the sex as long as it's consentual and fully disclosed.
So, a lie by omission is still a lie.