Refering to the buthole and how if your gay you would have sex through the buthole, going in. And the crap comin the other way would be out. Thus making it go two ways, a 2 way street. Straight people's butholes would be considered one way streets, because stuffs only goin out.
Teacher: Alright boys, im here to talk to you about homosexuality, most of your butholes are 1 way streets, but some may be 2 way streets.