So, after several months, I can tell that daily amino acids (a rather small quantity of them) still completely suppress post-orgasmic symptoms (I do not orgasm very often : usually once every 2 weeks, sometimes once every week - more AA might be necessary if I did it more often).
What is interesting is that if I catch a little pathogen which make me almost not sick (like an irritated throat...), an orgasm usually makes it worsen (and I get a real cold, etc.). It did it twice for covid, at least once for a flu, and many times for normal colds. It already did it in the past, but as I was sick with POIS too (usually - sometimes, precisely POIS wasn't triggered when I was already sick) it was harder to single out this effect. Now that I have zero POIS, I can see it clearly.
So I suspect that, at least for me, orgasm triggers a "transient immune deficiency", as nanna thought, even though the process may be different from the one he considered. Either some pathogen, or one latent pathogen in particular, or the immune response that follows, or the cause of the transient immune deficiency - consumes a lot of amino acids after orgasming (either directly or through ATP depletion, etc.), and the sudden deficiency produces "POIS symptoms".
Post-excitatory symptoms aren't linked to amino acids and still there, although taking zinc seems to prevent them to some extent (but not entirely). I suspect it's another consequence of the "original POIS cause", which may be linked to neurotransmitters, blood circulation, immune reactions.