It's not unheard of for someone to develop ME shortly after being with someone new, or even after a single sexual encounter.
I have heard this story many times over the years on my website, which details the virus I caught, and the array of mental and physical symptoms it precipitated.
People often report on my site that they caught their similar/same virus after a sexual or amorous encounter, frequently just through a one evening affair. They report they catch a respiratory or gastrointestinal virus, which causes a bad sore that or GI symptoms like diarrhea, and then often terrible mental symptoms ensue within weeks or months, like severe anxiety or horrendous anhedonia, as well as various physical symptoms.
This was also the route I caught my virus, just though kissing someone I met one evening. Woke up the following morning with a really bad sore throat, and the rest is history. Severe anxiety and anhedonia soon ensued, as well as increased fatigue, but it took a few years (and nasty episode of what appeared to be aseptic meningitis, likely from the same virus) before that fatigue developed into full ME/CFS.
That's when you start to think in terms of "if only". If only I had not gone into town on that evening...
I unfortunately, directly or indirectly, passed this virus on to 30+ friends and family via normal household social contact, a few of which then rapidly developed nasty cases of anxiety disorder or anhedonia. And nearly all the people who caught my virus showed some sort of permanent mental state changes, albeit subclinical mental symptoms, like becoming more irritable, grumpy, negative in outlook, losing motivation, mild anhedonia, complaining more of fatigue, and becoming emotionally flatter. Nothing that you could easily quantify, but certainly noticeable if you know the people well. And certainly symptoms which subclinically reduce the quality of life. My virus produced some unusual physical symptoms, so it was relatively easy to know who had caught it. The most severe physical symptoms were sudden heart attacks in 3 people.
All that misery, in myself and others, transmitted just from one kiss!
Of course when it comes to the ME/CFS that can appear after mononucleosis, remember that mononucleosis is not colloquially known as the "kissing disease" for nothing.