Ann Bishop Mullany lives in Baton Rouge. She is unhappily married to John, a successful lawyer, and has never experienced an orgasm. She is in therapy. Graham Dalton is an old college friend of John. He is now a seeming drifter who, after nine years, returns to live in Baton Rouge. Graham arrives to find Ann, who has no idea that John has invited Graham to stay with them until he finds an apartment. When John arrives home, Graham's demeanor becomes remarkably more guarded, due in large part to John's overt disapproval of Graham's bohemian persona. They also discuss the fact that Graham's college girlfriend, Elizabeth, is also living in Baton Rouge.
John is sleeping with Ann's sister, Cynthia, a free-spirited bartender. He rationalizes it by blaming Ann's frigidity. He frequently leaves his law office mid-day to meet with Cynthia, instructing his secretary to reschedule clients, even when they are already in the lobby waiting to see him. Ann makes an impromptu visit to Graham's apartment, where she notices stacks of camcorder tapes around the television. When pressed, Graham explains that he interviews women about their sexual experiences and fantasies, on videotape. Ann, overcome with shock and confusion, leaves his apartment.
Within a day, Cynthia appears at Graham's apartment and introduces herself. Cynthia presses Graham to explain what "spooked" Ann the preceding day. Graham explains the videotapes, and admits to Cynthia his sexual dysfunction: that he is impotent when in the presence of another person, and that he achieves gratification by watching these videos in private. Graham propositions Cynthia to make a tape, assuring her that no other person is allowed to see the tapes. She believes him, and agrees. Cynthia reports back to Ann, who is horrified. Cynthia also tells John, who also reacts very negatively (though more than a little possessively).
Ann and especially John are both reactionary in their condemnation of Graham, who in one conversation reacts with perhaps the defining line of the film: "I look at you, and John, and Cynthia, and I feel... comparatively healthy". When Ann discovers Cynthia's pearl earring in her bedroom (she knows it belongs to her sister since she had mentioned that she had lost it) while vacuuming, she is furious. She heads over to Graham's apartment with the intention of making a videotape. Graham objects, telling her it is something she would not do in a normal frame of mind. She insists and Graham relents.
Afterward, Ann demands a divorce from John. In the ensuing argument, John gleans that Ann has been to Graham's, and that she made a video. He goes to Graham's house, hits him and locks him out of the house, then watches Ann's tape. In it, Ann says she has never felt any kind of 'satisfaction' from sex. After Graham asks if she ever thinks of having sex with other men, she admits she has thought of Graham. Ann later turns the camera on Graham, who resists but she persists. Graham confesses that he is haunted by Elizabeth, and that his motivation in returning to Baton Rouge is an attempt to achieve some closure. He explains that he was a pathological liar, which destroyed an otherwise rewarding relationship with Elizabeth. He explains that he has since gone to great lengths to keep people at a distance and avoid relationships. Ann starts touching and kissing Graham; Graham turns off the camera; it is implied that the two have sex.
A chastened John joins Graham on the front patio and, with obvious pleasure, confesses to having sex with Elizabeth while she and Graham were a couple. But he also helps Graham see Elizabeth in a more realistic way. "She was no saint. She was good in bed and she could keep a secret. That's all I can say about her." and then leaves. This makes Graham furious and he goes into a rage and destroys all of the tapes, as well as his camera.
In the end, John is summoned to his boss's office, where itâs implied that he is about to be fired due to his frequent cancellations of meetings with important clients to the firm to have sexual trysts with Cynthia. In the next scene, Ann and Cynthia reconcile at the bar Cynthia tends before Ann returns home and joins Graham on the front porch, as they appear to be a couple.

Question: Who is Ann's husband having an affrair with?
Answer: Her sister, Cynthia.
Alternative answer: Ann's sister, Cynthia