Kissing beautiful celebs isn’t all it’s cracked up to be.Matt Damon and Emily Blunt in ‘The Adjustment Bureau’; Gwyneth Paltrow and Robert Downey Jr. in ‘Iron Man 2’Credit: Universal Pictures; Francois Duhamel/Industrial Light & MagicEven though it’s just acting, a kiss between actors is still a real kiss — unless you’re Jonah Hill and Lauren London in the film You People, of course.Otherwise, actors really do have to pucker up and act professio…