Nick Jonas Strips Some in Gay Bar

Nick Jonas learned, when in Rome ... because he joined the crowd at a gay bar with some half naked men and became somewhat naked himself.

Nick teased the crowd at BPM's Gay College Tuesday's in NYC ... coyly leading the excited crowd into believing A LOT was about to come off. He stopped short of the full monty, but still did not disappoint.

He was there to promote his new single but had the urge to jump on stage with a bunch of shirtless dudes.