JUSTIN BIEBER posted a sexy picture of himself making out with his fiancée HAILEY BALDWIN in a hot tub . . . but something about it didn't quite sit right with JOHN MAYER.
Because he commented, quote, "Interesting dynamic with the photographer in the hot tub. That's gotta feel super third wheel."
He added, quote, "I see a lot of myself in this hot tub photographer. Imagine being in a hot tub on business."
(There's no word who took the picture)