The Cleveland-Loretta Quagmire is episode 05 of family guy season 04. It was originally aired on June 12, 2005.
When Peter hears screams coming from Cleveland and Loretta’s house, he tries to help but discovers Loretta in a compromising position. Peter identifies the other man as Quagmire and decides to tell Cleveland of his wife’s extracurricular activities. But when Loretta walks out on Cleveland he doesn’t take it lying down.