Just Married

The film opens with Tom and Sarah in the airport, then flashes back from the moment they met up to the present. Working-class Tom Leezak and upper-class Sarah McNerney meet up when Tom accidentally hits Sarah with a football. A few months later, despite opposition from Sarah's rich family, they get married. They have kept a secret from each other: Tom doesn't tell Sarah that he accidentally killed her dog and Sarah doesn't tell Tom that she slept with Peter Prentiss, an old boyfriend and her family's friend, after she and Tom started dating.
Flying to Europe for their honeymoon, they attempt to consumate their marriage by joining the mile high club, but fail rather publicly. They arrive at their classy hotel at the foot of the Alps to find that Peter has sent them a bottle of cognac "with love", while Tom's friend Kyle has sent them a Thunderstick A-200 sex toy. Tom tries to force the toy's American plug into the European outlet and he shuts down the entire village's electricity. The newlyweds leave the hotel after Tom has a heated argument with the hotel owner and pays a large bill to repair the power. While trying to find another hotel they crash their undersized car into a snowbank, stuck until daylight and once again unable to consumate their marriage.
They make their way to Venice, staying at a pensione recommended by Tom's father. The pensione turns out to be a wreck, and they soon check out after a cockroach crawls over Tom when they tried to have sex.
The couple secure a nice Venetian hotel with the grudging financial help of Sarah's father. They go sightseeing, but Tom quickly gets bored and abandons Sarah so he can watch sports in a bar. Sarah runs into Peter, who is staying at their hotel on business. This prompts her to initiate a conversation with Tom in which he reveals that he accidentally killed her dog and she reveals she slept with Peter. The couple storm out of the hotel and each go their separate ways: Tom going back to the bar, where he meets American tourist Wendy, and Sarah going sightseeing, where Peter follows her. Wendy flirts and dances with Tom, who escapes through a bathroom window when he realizes she wants to have sex with him. He returns to the hotel, only to learn from the maÃ®tre d' that Sarah has gone out with Peter for the evening. Tom returns to the bar, only to be acosted by Wendy again. Tom tries to think of a clever way to get out of his situation, and finds himself tricked into walking her to his hotel room, where the girl rips off her top before Tom blurts out that he's on his honeymoon, upon which the girl finally leaves.
Sarah gets drunk so Peter takes her back to the hotel. When he kisses her at the entrance, she slaps him and reminds him that she's on her honeymoon. Tom sees the kiss from the balcony but not the slap. When Tom confronts her in their room, Sarah finds Wendy's bra. Peter bursts in to ask Sarah to run away with him to Seattle, leading to a fight that lands Tom and Sarah in jail â still without consumating their marriage. Peter bails them out and the couple angrily decide to go home to Los Angeles, returning to the opening moments of the film.
Sarah has moved out and Tom wants to get back with her. Upon receiving advice from his father, Tom attempts to see Sarah at her family's estate, but gives up after unsuccessfully trying to ram the gate. However, Sarah opens the gate herself after seeing Tom make a romantic speech to the camera and the two rush out to proclaim their love for each other. Sarah's family finally accepts Tom and Sarah's relationship.

Question: Who does Sarah reveal she slept with?
Answer: Peter