I remember a guy talking about how he didn't realize his anus could be a source of sexual pleasure until a girlfriend he was with swiped during a blowjob. While I've had a situation where a girl got my asshole in a similar situation…
How can a person NOT realize their anus could be a source of sexual pleasure: Hasn't everybody felt some kind of pleasure or tickle when they were washing themselves in the shower or in the tub?