Ryan Seacrest's GF -- Kissing Another Man

Ryan Seacrest's girlfriend Julianne Hough passionately locked lips with another man in Florida yesterday - but good news for the "American Idol" host ... all this took place on the set of her new movie.

Julianne and Diego Boneta co-star in the upcoming Tom Cruise movie "Rock of Ages" - and if it was not clear from photos, beach bully ... they play a couple very buddy-buddy.

So far, no word Seacrest - but we're guessing ... That must hurt.