One would have to assume of course that POIS IS a semen allergy problem.
I'm beginning to believe that semen allergy COULD upset some other system, as it has been shown that long term allergies of various types have unbalanced hormonal and neurotransmitter systems.
That said, I think several other things could also upset the same "systems" which ever they may be.
The GAP, if that's it, seems very illusive. There are a number of places it could be and also a number of circumstances that could cause it, from physical to hereditary. Yet there are few, proportionately, who have it.
Even with things like vasectomies, they consider the "encroachment" caused by such surgery as not very likely to infringe upon the famous blood barrier. So it seems very well protected, and that could explain why the numbers are low.
I've probably had more damage done by the famous crossbar and slipping on a pedal, than from the bike seat. But my POIS didn't start till very many years after those incidents, and much closer to the vasectomy reversal several years back now.
(Seems like everything is "several years back" these days, sigh).
So, for what it's worth.