When commending Nick for breaking up with Marjorie—whom he views as inferior to Nick—Bill uses a metaphor to explain why they were not a good pair:
“You came out of it damned well,” Bill said. “Now she can marry somebody of her own sort and settle down and be happy. You can’t mix oil and water and you can’t mix that sort of thing any more than if I’d marry Ida that works for Strattons. She’d probably like it, too.”
The metaphor here—“You can’t mix oil and water”—is commonly used to capture the ways that two elements don’t mix well together. Due to the different chemical makeups of oil and water, they cannot combine. Bill is using this metaphor to (in his mind) comfort Nick, and assure him that he did the right thing in ending his relationship. In saying that Marjorie should “marry somebody of her own sort,” Bill is clearly implying that he believes her to be inferior to Nick, possibly due to a lower socioeconomic status.
What is unspoken here is the possibility that Bill’s disregard for both Marjorie and Ida (who he seemingly used to date) is due to his disillusionment with marriage generally. As a member of the Lost Generation who came of age during and after World War I, Bill is clearly frustrated with traditional social expectations. He doesn’t want to get married, settle down, or find a job that he dislikes. His frustration may partially be due to classism, but it's likely also due to witnessing the trauma of the first global war and feeling aimless in its wake.
