Robert Pattinson's biggest fear has been realized.

On July 17 in L.A., the 26-year-old actor's girlfriend and Twilight costar, Kristen Stewart, 22, was caught by photographers in a series of steamy rendezvous with her Snow White and the Huntsman director, Rupert Sanders, 41 --  and the new issue of Us Weekly has the exclusive photos and details.

Sanders -- married to British Vogue model Liberty Ross, 33 -- has two children, ages 7 and 5. During his marathon makeout session with Stewart, "it seemed like they couldn't get enough," according to one photographer.

Reps for both Stewart and Sanders had no comment.

For more details on Stewart's affair -- from her initial interactions with Sanders to how her infidelity will affect her Twilight Saga: Breaking Dawn -- Part 2 promotional duties with Pattinson -- pick up the new issue of Us Weekly, out Friday!