This is indeed very interesting! Please research this further!
Since prolactin increases after O, it is often implicated in causing POIS. But maybe we've got it all wrong and prolactin is not going up sufficiently.
By the way, this might be related to the difference between masturbation and sex: I believe that my sides are way less when I masturbate in a way that brings my heartrate up long before orgasm.
I have the suspicion that an orgasm can have bad effects when heartrate is low/normal because in that case the orgasm might create a shock in blood pressure.
Basically what I do is I stand, use both hands to make a o-shape, and then thrust into my hands without moving the hands. This requires way more energy than how I used to masturbate, but it has the effect that my heartrate goes up first.