Hi Spartak,
Some members have reported POIS symptoms with only pre-cum fluid ( what is coming from the Cowper glands, also called bulbourethral glands). So, it is not necessarily about complete semen - there can be allergen in other fluids, like the pre-ejaculatory fluids. And a few drops is hard to detect.
Some members have reported symptoms with even some minor level of arousal, like seeing an attractive women in the street ( not a lot of symptoms, but still, a few). Is there some minimal production of pre-cum that would even not show outward? I don't know. If not, there could be some purely neurological trigger for POIS, at least for a part of the symptoms, and another part from allergic origin, who knows. But it is a complicated issue, and not all cases are identical. In my case, when I had no relief method at all, POIS following a NE was 10% to 20% , compared to a 100% POIS after "awake" ejaculation. Go figure. Even from one "awake" E to another, I experiment different levels of severity, with no clear reasons.