Butt Compass
Butt Com•pass (verb)

An occurrence of such a state in the penis of a male, when the penis becomes erect, and causes the the male to turn freely on a pivot, pointing in the direction of another male’s butt.
Example: His moral compass told him that his attraction to another man’s butt was wrong, but his raging butt compass confirmed his sub-conscious attraction to the man in the spandex bicycle shorts.