When a guy is arousedblood will flow to the genitals. As this happens muscles tighten in the pelvis. This results in an increase in fluid going to the genitals and a decrease in fluid leaving and can cause a rise in pressure. The result is an erection. After an orgasm the muscles relax and the fluid levels balance.
If a male is aroused for a long period of time and does not orgasm, cramping can occur. This is due to the build up of fluid and may also be known as blue balls. Blue balls occur when the fluids stay in the genitals for a period of time. The cramping often feels like heaviness in the genitals, an aching or a slight discomfort.
The pain will usually subside within an hour on its own. Although guys may complain about the cramping, most experts agree the pain is not a big concern. If it becomes really unbearable, the male should seek medical help.