What is that you ask?
The idea that lying is always immoral.
(Now if I wanted to make my life easy, I'd just talk about lying to save lives. But I don't want to make my life easy.)
Here are two considerations to take into account:
There is a spectrum of how much people want to know stuff.
At one end of the spectrum: "Are you cheating on me?" "Do you have HIV?"
At the other end of the spectrum: "How's your day going?" "How was your trip?"
If I just had horrible diarrhea, and you ask me, "What did you do today?" I'm allowed to say, "Not too much, I just watched some TV." I don't have to tell you about the particulars of my bowel movements. I don't even have to merely omit it. I'm allowed to claim or imply that I didn't just have a horrible bathroom experience. Why? Because you don't particularly want to know, and my preference for you not to know is stronger than your preference to know.
There is also signaling balancing.
You have to balance the accuracy of the literal thing you are saying with the accuracy of the thing you are signaling. For example, if my grandma asks me how she looks, if I don't tell her she looks great, I have signaled I don't care about her feelings which is inaccurate. Sometimes literal statement truth has to be traded-off against signalling accuracy.