Don't worry about your question; it is just physiology. Was it longer than 2 hours after your bike ride? Riding puts pressure on everything and as mentioned, a good seat and shorts can help. Usually what is impacted is the erection but as that doesn't seem to be the case, it could have been from decrease nerve sensitivity (from riding vibration) or because your prostate was inflamed, which supplies most of the goods for the end result. I can't say if this will always happen after cycling but if it happens when you are not cycling, I would get checked for prostatitis.
I think the best seat has a cutout to decrease pressure on the perineum...if you google perineum and biking you will find more information.