Making Love (1982) 

Claire and Zack have been happily married for eight years. She is a successful TV executive, and he's a prominent doctor. They share a consuming passion for the music of Gilbert & Sullivan, are sexually faithful to each other, and have just bought a dream house. But, something alarming is eating away inside Zach. When Bart comes to his office, the two seem to connect. Bart thinks the lump on his neck is a tumor; it turns out to be a false alarm. Doctor and patient then go out for lunch and hit-it-off. Weeks later, they have dinner and end up at Bart's house. Zack's curiosity is quenched when they make love. Zack decides soon thereafter that he must leave his wife.