When one has become so obsessed with sex that every sound sounds like people having sex. This is usually caused by the suffering indivdual being sex deprived or having not had any sexual activity for an extremly extended period of time.
As Johny walked thru the halls of his dorm he thought he was hearing sex from behind each door. Infact no one, anywhere, on campus was having sex or even watching porn at the moment. Johny simply had not gotten any in over a year and he was now suffering from sex paranoia.