Whipped
(2000)

Over the course of a few weeks, Brad (Brian Van Holt), Zeke (Zorie Barber), and Jonathan (Jonathan Abrahams) are each convinced that Mia (Amanda Peet) loves them, and their friendships are on the verge of collapse. But suddenly, as the trio are having their weekly Sunday meal at the diner with Eric (Judah Domke), Mia comes in with three Italian cyclists and gets real friendly with all of them. The guys get her attention and voice their displeasure over it, but she casually tells them that she will talk to them later in the week, essentially coming off like she is a player herself, and then they watch her leave with the Italians.

However, despite what she said, the guys never saw or heard from Mia again after that, and they all got back into their old lifestyles before they met her: Zeke resumed getting robbed once a week by the girls he took home, but this time he just watches them do it. Eric was still trying to find creative ways to have sex with his wife so he could still remain a part of their group, and he is shown starting up an egg beater while his wife screams. Jonathan resumed his masturbating life, and Brad continued on with being a player. However, their friendship was not the same as before, and their weekly Sunday get-togethers began to taper off until they eventually stopped. 

Mia is then shown eating with a group of her friends and badmouthing all of the guys and how they were in bed (though she gives a compliment to Jonathan about the size of his penis), and she reveals that, when Eric earlier met her in private to voice his concern that she was causing the group to split apart, she performed oral sex on him. She also reveals that she was there in the park when the guys minus Eric were attempting to pick up three girls there earlier in the film and knew right off the bat that they were players, so she decided to play them all just to watch them squirm and see their egos get deflated and end their friendships. She has no qualms about what she has done, believing that everyone screws everyone in the end.

Thanks Tornado Dragon!