Mona is attracted to Kramer, which Jerry, Elaine and George cannot understand as she has never been with a man before. This angers Susan, due to her dislike of Kramer, as well as George, who worries that he drove Susan to lesbianism.
Mona's romantic interest in Kramer confuses Jerry, Elaine, and George, who have never seen her interact with males. This confusion leads to Susan's anger because of her dislike for Kramer and George's worry that he might have influenced her sexual orientation.