Steffi, I don't think you need to be a mind reader. Right after the extract you just quoted, I gave an example: cheating on your wife. Anyone can "omit" this piece of information to a SO if it happens, but can one honestly sustain that it's not lying? It seems a stretch to pretend that one could know so little about the person s/he is living with that s/he couldn't think of anything the SO would want to be informed about. If one really needs a crystal ball for that, I suppose s/he could as well live with a stranger.
And another thing about this thread: we are mostly talking about the characterization of omission vs. lying, this was after all Susan's initial question. Nobody is telling anyone what to do in one's life. I think people get mixed with the two. It seems to go this way: if you tell me it's not an omission but a lie, then you are telling me I am a bad person. As far as I am concerned I don't. I am just asking fellow members to own the fact that some omissions are equivalent to lies in the SO's perspective, and that it doesn't take rocket science to figure out which ones are important. Then one does what s/he wants.
-- EDIT --
I know the "cheating on wife" example will trigger some. There are of course all levels of omissions that would be considered lies by a SO. Cheating is just an obvious example.
