Britney Spears -- MAKES OUT WITH OTHER MAN ... Boyfriend's Cool With It

Britney Spears sucked face with a dude who wasn't her BF yesterday ... but her man wasn't upset, because it was all part of the script.

The spit-swappin' all took place on the L.A. set of Britney's new music video. The identity of the male kissing partner is unclear ... but he appears to be pretty good looking.

(UPDATE: Our friends at toofab.com have ID'd the dude -- click here to find out more!)

As for Brit's BF David Lucado -- no word if he was on the set ... but if you knew your girlfriend was going to make out with a younger, hotter man -- would you wanna be there to watch??