No, it would be hard to test how and if cortisol goes up or down for us. Even if one of us checks cortisol right before and right after orgasm, and if we we notice it goes up, maybe it doesn't go up sufficiently. Let's say it goes up right as much as it does for healthy people. Then maybe it doesn't go up at the right time - 10 seconds, or 1 minute, or 10 minutes after orgasm? No way to test this on our own, this can only be examined in a lab in a controlled experiment. This can be tested if for a healthy reference group and for us, cortisol is measure before O, as well as specific times after O: a few seconds, a few minutes, one hour later, one day later, two days later. My speculation is that for us, it doesn't go up as it should be (or at least not enough up).
No, the horse anecdote doesn't contradict - rather supports - the speculation that we POISers don't get enough cortisol after ejaculation. I am not saying that we are incapable of producing cortisol whatsoever. In fact, my POIS that week would be due to low cortisol, but then the horse ride triggered its release, hence the quick recovery with the horse ride.
I am not saying cortisol is the route cause. I am only saying that cortisol is involved.
The reason I am thinking about cortisol is that I am analyzing the mercury toxicity hypothesis proposed on this forum. I've read that after physical exercise, mercury toxic people have cortisol levels go down while healthy people have them go up, and , as a result, mercury toxic people have fatigue after more intense physical exercise. I view orgasm and physical exercise in parallel here, since I get POIS symptoms also after more intense physical activity.