A polt twist toward the end of The Great Believers, but is it believable?

Nearing the end of Rebecca Makkai's 2018 novel, The Great Believers, and it becomes apparent - spoilers to follow I guess - that the main character, Yale, will, like so many of his friends and colleagues ca 1985, die of the AIDS virus - though the novel has not yet said so directly. But after we thought that Yale was off the hook so to speak despite his former lover's infidelities and subsequent infection - Yale gets a clean lab report and is relieved though not ecstatic, as the rest of his life is falling into a shambles - he gets/we get a curve ball: Turns out that the grad-student intern w/ whom Yale is now engaged in an affair of casual, noncommittal sex, Roman, has been a big promiscuous player in the Chicago gay scene. I could accept this twist if it were not so entirely a shift, even a violation, in Makkai's portrayal of Roman, albeit through the POV of Yale, who had assumed (why would he?) that Roman was virginal and quite inexperienced in the homosexual scene,a  repressed Mormon, no less, who was unsure of and ashamed about his sexual orientation. So I get it that Yale's character judgment was off - though he seems to be shrewd, cautious, and sophisticated man - but I feel a bit yanked around by this - it's not so much a plot development as a sleight of hand pulled off by the author. In any event, it looks likely that Yale has been infected by Roman and he's headed toward another untimely and unnecessary death. I think another hint was dropped in the 2015 subplot, i.e., 30 years later, in that Fiona's granddaughter may be named in Yale's memory. Still, Makkai is being cagey right to the end - good writerly strategy! - and she may pull yet another trick out of her hat, so to speak.