While searching for a mechanism to explain the symptoms of POIS, I came up with a novel idea.
I learned that milky or cloudy urine usually comes from the lymphatic system via the kidneys. The lymphatic system drains off excess fluids in the body, so semen could be getting into our lymph due to some physical abnormality in the reproductive system. How this translates into all the other symptoms we experience in POIS, I am not sure yet.
However, that would explain why many of the remedies described on these forums are also things that help cleanse the lymphatic system. This includes exercise, bromelain, quercetin, deep breathing, laughing, dietary changes, saunas, garlic, sleep, and hot/cold showers. Vasodilation increases lymph flow.