The stanza serves to point up Hazel's "outsiderness." The term roommate is casual collegiate terminology like coed. Casual for Shade, who, as an oldster, is separated from student life. As casual as the use of the term would be to a Timofey Pnin, who had long ceased to be too much aware of students on the campus.
This is why Shade's choice of the word roommate has been accepted without question for 40 years. Not because millions of readers around the world have missed a clue to Shade's presumed philandering nature.