a forest fire happens when someone with excessively long pubic hairs fucks for too long and too hard, way past the point of bush burn. Forest fires occur when the pubic hairs become so heated by friction that they combust, resulting in a very un pleasent dilema.
"hey, i heard you give it to a woman hard, you wanna head up to my room"
"yeah well let me shave my shit first...don't want another forest fire happenin'.