Final answer:
Myrtle has an affair because she believes she can have more fun with a man she can never marry.
Step-by-step explanation:
Myrtle's reason for having an affair is that she believes she can have more fun with a man whom she knows she can never marry. She sees the affair as a way to escape the limitations of her marriage and experience excitement and happiness. She mentions this to John, who becomes angry and confronts her about her infidelity.