A warm fuzzy occurs when a male engages in sexual intercourse, with a female who has neglected to shave her labia majora. As a result, the males shaft becomes irritated and causes slight increase in temperature, which conveys the "warm fuzzy" feeling.
John: Hey man, how was doing her the other night?
Bob: It was ok, but she didn't shave her box, so I got a warm fuzzy. Other than that, it was sweeet!