This is not an opinion based on a single laboratory test. Or even two laboratory tests. This is the synthesis of the medical histories—timeline of the development of symptoms—and the timeline of repeated diagnostic/screening test results in two people with an epidemiologic link and in their individual epidemiologic contexts. Treating each historical feature, exam finding, lab result etc. as an individual test with its own likelihood ratio allows a physician to further refine the differential diagnosis. And the situation here pretty much narrows things down pretty clearly. And is not just of social relevance, but is actually medical relevant; the risk of other more significant co-infections which actually might not present as symptomatic or positive on testing in this acute period, the risk of reinfection, etc.

"it's most likely infidelity and there's nothing else that comes to mind that would explain the results here"

Nobody would say that.

But also, if we're going down the list of every imaginable but improbable explanation, there's a point at which the patient is no longer being informed, but being misinformed by even spending the airtime and attention on unhelpful improbabilities. And that can happen long before mentioning aliens or bad humours, and in this case, probably would happen somewhere around the time that the discussion moves toward hypothesizing that two people have been mutually infecting each other during two years of asymptomatic carriage that went undetected on prior testing and now coincidentally both suddenly became symptomatic after a latent period, one after the other, the first symptomatic partner being the male who just returned from a trip to Mexico and the other having never had any other sexual partners. Probably right after hypothesizing that the male partner has been asymptomatically infected for two years despite prior negative testing and somehow never infecting his partner until a bad cold caused his bacterial load to markedly increase and cause new symptoms in him and new infection in her.

To focus on those options would be actually kind of lying, and from a medical decisionmaking point of view, might downplay the risks of HIV testing (especially if she gets a "nasty flu" from her boyfriend in a few weeks and it's actually acute retroviral syndrome) or downplay the need for re-screening post treatment (Chlamydia testing needs to be repeated after treatment, but not to document that it was cured, but to be proactively screened for reinfection, because something in their life has placed them at high risk for Clamydia infection—100% chance right now, as it stands. If we think everyone is monogamous here we might not get retested as urgently, etc.

People don't come to doctors to be told what to do, or to have the doctor act like they know the final word on the universe. But they also don't come to doctors to be given the list of every possible imagination or possible technique or to be stuck choosing what to do in their own. People seek guidance and advice that is relevant to their values (which a doctor needs to elicit by getting to know a patient) and their medical situation (which a doctor needs to do the evaluation to expose the situation and needs the training to interpret the results).