Frasier reacts in horror when Martin shows him a picture of Artie's wife and her huge butt, but earlier Frasier and Niles discuss their memories of cookouts at Artie's house when they were small boys. Seems odd that Frasier would not have noticed or remembered if Mrs. Walsh had such a huge posterior.
Frasier: Roz, he's probably all of 19.
Roz: That's legal.
Frasier: Well, koo koo ka choo, Mrs. Robinson!
Frasier taunts Roz with a line from Simon & Garfunkel's song "Mrs. Robinson", which has become synonymous with the movie The Graduate about a young man having an affair with an older woman.