ironic promise ring
This is the ordinary "promise" ring: worn by an unmarried teen girl to signify that she will abstain from sex until married. In this case, the ironic promise ring is worn by a girl either as a cover-up for sexual activities, or in the pure ironic sense. In either case, she's doing it!
Sharon and Bill were fucking like bunnies; but she masked that fact by wearing her ironic promise ring.