The condition commonly known as blue balls in males. It most commonly occurs when a man is continually aroused but never actually ejaculates. It is caused by an excess build up of fluid from the prostate gland (the fluid that makes up semen).
I have also heard that it was a build up of stopped blood in the testicles and penis.
My wife and I were making love when the kids came home early and we had to stop. Man, had some bad prostatic congestion (blue balls)!!