Yes, I know what you mean -- it's that thing of treating details of sexual preferences, whether it's Fay's "fetishes" or Marq's thing for bitten fingernails/claws in Stars In My Pocket Like Grains Of Sand, with seriousness and weight, as something important, and -- I am struggling for the right words here, but something like "existentially valuable", maybe?

"Valuable" works for me. Not treated as a quirk or an add-on, not as even merely valid, but an essential part of the person and at the same time, in keeping with the rest of the novel, requiring of no special explanation.