When, during vaginal intercourse (or the attempt at) in the missionary position, the male's lower umbilicus area (the area below the belly button) as well as the surrounding abdominal region, are penetrated by the female's stubbly, three-day-beard-like pubic hair. The repeated, thrusting movements of the male eventually lead to "rugburn" - conventionally a reddish, rash-like discomposure of the skin. Warning: In certain cases, rugburn can yet create skin ruptures and cause minor bleeding.
"Bro, look at my lower stomach area. It's completely swollen and dark red. Golly, I knew having vaginal intercourse with Aunt Shelly would be wrong in many ways, but I never thought she'd give me rugburn."