(I’ll break my own rule here and use the more faithful title instead of Cities of the Plain.)
For all initial appearances, Sodom and Gomorrah is more of the same. There is not the break that there was between past volumes: the transition from the flashbacks of Swann’s Way to Proust’s youth, the transition from Balbec to Paris. We are still in high society and, with the exception of a brief return to Balbec, the scenery is numbingly similar to that of The Guermantes Way. The apparent shift is mostly psychological, as Marcel has once more receded from the picture now that his infatuation with the Guermantes has expired. But there is a more subtle thematic shift–a nascent one–that hints at some resuscitation of life from the nihilistic attitudes of the prior volume.
Sodom and Gomorrah is divided into two parts: the first is thirty pages, and the second is five hundred. Without reading too much into this (Proust hadn’t finished revising the thing, after all), I’m still inclined to give equal weight to each. While the second is society-play of the sort seen before, the first is something else entirely. Marcel surreptitiously spies on M. de Charlus and the tailor Jupien, several times. They speak of homosexuality and engage each other as though belonging to a secret society, not as lovers but as conspirators, speakers of gossip who are allied with one another because of their kind. Proust makes a few observations about what he calls this breed of “man-women” which fall somewhere between stereotyping and species classification, but what’s more interesting is the process by which this discovery is made.
Marcel, in the novel, has never before eavesdropped on people to this extent; Proust’s omniscient knowledge of others has been treated as a given. Here, then, is something that has been consciously excluded from the novel so far. Homosexuality has surfaced at times (Odette, Albertine, Saint-Loup), but not as a principle of social order. In the women it was a dalliance; in Saint-Loup something to be rejected and ignored. But in Charlus, it’s a character factor that is so fundamental to his personality and his behavior that it, for Proust, separates him into another species:

This single new notion will bring about an entire regrouping, thrusting some back, others forward, of the fractional notions, henceforward a complete whole, which we possessed of the rest of the family…Now the abstraction had become materialized, the creature at last discerned had lost its power of remaining invisible, and the transformation of M. de Charlus into a new person was so complete that not only the contrasts of his face and of his voice, but in retrospect, the very ups and downs of his relations with myself, everything that hitherto had seemed to my mind incoherent, became intelligible, appeared self-evident, just as a sentence which presents no meaning so long as it remains broken up in letters arranged at random expresses, if those letters be rearranged in the proper order, a thought which one can never afterwards forget. (636)

The way Proust conceives this discovery, as an organizing principle, suggests that underneath the trivial chaos of social conceits, there are powerful and submerged forces, of which Marcel had not previously been aware. And permanence is a key aspect of his conception of homosexuality: for many pages, Proust has spoken of the fickleness of emotion and the alienation of people from their own past emotional states, but homosexuality, he implies, is something entirely different, a trait that underlies fundamental character, not a fleeting whim, and so is something that can be looked to for reference about a person’s deeper nature. Proust says nothing of this directly, but the contrast is difficult to ignore, and this treatment of homosexuality as more fundamental than love or any other emotion suggests that there may yet be some meaning to be found in the detritus of high society.
Of course, Proust then has this to say:

These descendents of the Sodomites…have established themselves throughout the entire world; they have had access to every profession and are so readily admitted into the most exclusive clubs that, whenever a Sodomite fails to secure election, the black balls are for the most part cast by other Sodomites, who make a point of condemning sodomy, having inherited the mendacity that enabled their ancestors to escape from the accursed city. it is possible that they may return there one day. Certainly they form in every land an oriental colony, cultured, musical, malicious, which has charming qualities and intolerable defects…I have thought it as well to utter here a provisional warning against the lamentable error of proposing (just as people have encouraged a Zionist movement) to create a Sodomist movement and to rebuild Sodom. For, no sooner had they arrived there than the Sodomites would leave the town so as not to have the appearance of belonging to it, would take wives, keep mistresses in other cities where they would find, incidentally, every diversion that appealed to them. They would repair to Sodom only on days of supreme necessity, when their own town was empty, at those seasons when hunger drives the wolf from the woods. In other words, everything would go on very much as it does to-day in London, Berlin, Rome, Petrograd or Paris. (655)

I’m tempted to just shrug this off as part of Proust’s general misanthropy, but it’s notable enough (especially next to his treatment of the Dreyfuss scandal, which I haven’t discussed at all yet) that it deserves mention, even without comment.