In a single sentence, the speaker tells us about a betrayal and, just as sharply, about their own part in it. The title, "Cheater," gives the verdict upfront, but the line itself is more interesting than the label. The speaker doesn't say they found out or were told. They say they "stayed until" they noticed the lipstick, which suggests they may have sensed something wrong long before and kept hanging on anyway.
That small word "until" carries the feeling. It hints at doubt, denial, and maybe a quiet hope that the suspicion was wrong. The lipstick on the collar is an almost old-fashioned piece of proof, so plain that it can't be explained away. The tone is cold and flat, with no shouting or tears, as if the speaker has finally run out of excuses on the other person's behalf. The mood is bitter, tired, and a little self-reproachful. The real sting is that it took something this obvious to make them leave.