There isn’t the frustrating stasis of The Guermantes Way in Sodom and Gomorrah, but nor are there the high moments of lyricism (as with the death of Marcel’s grandmother) or a central, unifying concept (as with Marcel’s obsession with the Guermantes). The volume is livelier, but it is more shapeless, at least until the final chapter.
Having brought up Charlus’s homosexuality and theorized about the secret world of “sexual inverts,” as Proust calls them, Proust is content to let the theme recede for large segments of Sodom and Gomorrah, preferring instead to recollect parties, receptions, and another, far less interesting trip to Balbec. While the proportion of dreary social events to interesting and novel portrayals of homosexuals is probably accurate in relation to Proust’s actual life, the volume sags when it should be advancing forward. And correspondingly, Sodom and Gomorrah is most involving in the two plots that involve homosexuality: Marcel’s romance with Albertine, whom he suspects may be a lesbian, and Charlus’s obsession with his protege Morel.
Charlus and Morel: Morel is a vain, obnoxious young musician, and Charlus has great affection for him, taking him under his wing in a similar manner as he did Marcel in The Guermantes Way, and then some. Charlus’s obsession with Morel is pathetic, but it’s also overtly comical in a way that Proust hasn’t previously allowed. In Swann’s Way, Odette was far more coarse than either Charlus or Morel, but she never lost control as Charlus does periodically:

His explosions of rage were too frequent not to be somewhat fragmentary. “The imbecile, the scoundrel! We shall have to put him in his place, sweep him into the gutter, where unfortunately he will not be innocuous to the health of the town,” he would scream, even when he was alone in his own room, on reading a letter that he considered irreverent, or on recalling some remark that had been repeated to him. But a fresh outburst against a second imbecile cancelled the first, and the former victim had only to show due deference for the fit of rage that he had occasioned to be forgotten, it not having lasted long enough to establish a foundation of hatred on which to build. (678)

Charlus’s freak-outs never get old for me. But the broad humor obscures what I think is the most important point here: Charlus’s inconstancy in his reactions, his abrupt fickleness, is a reflection of the same capricious nature that was shown by Swann’s affections towards Odette. Charlus is not exceptional; he is demonstrative.
The humor extends to burlesques. Charlus challenges Morel to a duel in a fit of pique. On hearing that Morel is going to a brothel, he arranges to spy on Morel in the act, but Morel is told at the last moment, so Charlus only observes him frozen by fright among the women. By themselves these events are trivial, but taken as another slant on Swann’s tragic pursuit in the first volume, they undermine the emotive force of Swann’s journey while reinforcing the (arbitrary) causative forces. The effects, as expected by now, are cynicism and despair.
The shift in tone is, at points, linked directly to Proust’s perception of homosexuality. Consider:

The invert brought face to face with an invert sees not merely an unpleasing image of himself which, being purely inanimate, could at the worst only injure his self-esteem, but a second self, living, active in the same field, capable therefore of injuring him in his loves. (951)

Proust is vague about the connection, but I take it to be this: homosexuals possess a secret that puts them out of conformity with society even as they superficially conform to its mores to the utmost. It is the awareness of this secret, and this dissonance with society, that makes them hyperaware of their own selves, and their own selves as reflected in others, who they are preternaturally inclined to see (a) as more similar to themselves (as “a second self”), and (b) as arbitrary agents. They are therefore more inclined towards arbitrary behavior and more likely to take it over the top. It’s a narcissism that is paradoxically directed outwards, since it causes them to ferret out the similarities of their surroundings to themselves.
Against this backdrop, Marcel’s romance with Albertine plays out under laws that strip it of the nostalgic, but myopic obsession that Swann had when he pursued Odette, and cast it in blank gray tones.