21. Shame (2011)
It's got all the makings of a blossoming romance: After casual flirtations in the office, co-workers Brandon (Michael Fassbender) and Marianne (Nicole Beharie) decide to make an official date. He shows up late (he stopped to watch a couple have sex through a hotel window) and all that office banter comes to a screeching halt when they realize they don't know what to say to each other.
Marianne, recently separated, expresses her hope for finding a lasting relationship. Brandon, a sex addict, has never had a relationship last longer than four months. Despite the awkwardness, Marianne allows him to whisk her away for an afternoon quickie the next day during work. Except Brandon's having trouble getting his equipment to work, so Marianne leaves.
Moments later, we see him having sex with another woman in the same hotel room. Pity those kids couldn't work it out.