The Rules of Attraction

Set at the fictional Camden College in New Hampshire, the film opens at the "End of the World" party, where students Lauren Hynde, Paul Denton, and Sean Bateman give apathetic interior monologues on their lives and briefly exchange glances with one another. Lauren, previously a virgin, takes a film student upstairs to have sex, only to wake up being raped by a townie while the film student records it; upon waking, she reflects on how she had planned to lose her virginity to Victor, her now ex-boyfriend. Meanwhile, Paul tries to have sex with a jock, only to be bashed when it turns out he is deeply closeted; additionally, a bruised and beaten Sean tears up a purple letter, before approaching and having sex with a blonde girl at the party.
The plot then moves backwards several months and explores the love triangle between Lauren, Paul, and Sean. Misinterpreting Sean's friendliness, Paul mistakes him for a homosexual and makes several advances to which Sean is apparently oblivious. Concurrently, Lauren also finds herself attracted to Sean despite saving her virginity for her traveling boyfriend, Victor. Sean reciprocates her feelings, and assumes the anonymous, purple love letters he has started receiving are from her.
While Paul is visiting his friend "Dick", Sean is seduced by Lauren's roommate Lara, who tells him that Lauren isn't interested in him because she has a boyfriend; despite having sex with Lara, however, Sean regrets it and realizes that he is in love with Lauren. It is then revealed that another, unnamed cafeteria girl is the author of Sean's love letters; after seeing him leave the party with Lara, she sends him a suicide note before cutting her wrists in the dorm bathtub. Lauren, finding Sean with Lara, runs to the girls bathroom in anger, only to find the unnamed girl's corpse, leaving Lauren extremely distressed. Sean, still believing Lauren wrote the purple letters, misinterprets the suicide note and assumes Lauren never wants to be with him.
After numerous failed attempts at suicide, Sean then fakes his death and, unaware that Lauren recently found a corpse, unintentionally upsets her further when she finds him pretending to be dead. After stealing drugs from dealer Rupert, Sean tries to speak to Lauren again, only for her to brush him off angrily. Finally happy now that Victor has returned to Camden College, Lauren is completely distraught to find Victor has no idea who she is.
Paul, upon finding a drunk Sean, tries to talk to him, only for Sean to reject him, causing Paul to run off upset. Sean then finds that the love notes have stopped, and is beaten by Rupert and Rupert's Jamaican partner, Guest. The protagonists then attend the "End of the World" party and the plot returns to the introduction. After seeing Lauren heading upstairs with the filmmaker, Sean finally accepts he cannot be with her, and tears up one of the purple letters he believes to be from her. It's then revealed that, rather than having sex with the blonde girl as he does in the intro, Sean was merely fantasizing, and he instead leaves his drink and exits. Paul and Lauren then meet on the house porch and reflect on the events of the movie and on Sean, who is seen leaving on a motorcycle. Sean then begins narrating his thoughts, only for them to end prematurely.

Question: What does Sean regret having with Lara?
Answer: Sex