From my opinion :
60% probability : allergic reaction to semen.
40% probability : hormonal/neurotransmitter depletion, especially dopamine. Semen is rich in catecholamines.
I can't believe in the allergic reaction hypothesis because :
1 : The semen is already in the body..all time
2 : Most off people doesn't have any classic sign of allergic reaction
3 : Many people doesn't need to ejaculate to get POIS symtoms
4 : Many people doen't even need to be aroused(or any sexual related activity) do get POIS Symtoms ; I tried full abstinence, it doen't solve the problem.I think we have a problem that somehow is making worse(or more visible) after O.
Still, I'm really not sure about anything...
I keep reading/watching very interesting stuff about medecine...I believe many people have some "general unbalance caused by our modern life style". POIS'er are just some of them that notice links with Orgasm.
If you have time watch this video, it's really interesting...A lot of people(International first class MD's) says right now that it will be really soon a change of paradigm in medecine...A kind of really simple medecine that doesn't brig back tons of money to pharmaceutical industry.
http://www.youtube.com/watch?v=-z3kRDYcvhA