Ryan Seacrest's GF Kissing Another Man

5/23/2011 6:15 PM PDT

[PHOTOS] Ryan Seacrest's GF Julianne Hough -- 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 ... it all took place on the set of her new movie.

Julianne and Diego Boneta co-star in the upcoming Tom Cruise flick "Rock of Ages" -- and if it wasn't clear from the manhandling beach photos ... they play a VERY touchy-feely couple.

So far, no word from Seacrest -- but we're guessing ... that's gotta hurt.