A very attractive young male, that one fantasizes about.
Taken from the 1940's when sexy women were glamorized, to where men would take their photograph and pin them up on the inside of lockers, or carry snapshots in their wallets.
Pin Up Boys are attractive or glamorous males, usually models or actors, that one enjoys dreaming about, fantasizing about in romantic situations.
These are basically males that are pleasing to the eye, draw attention to their looks, their bodies.
Pin Up Boys are those types that elicit a sexual attraction, or desire. They help to arouse a person, and foster fantasies of a sexual nature.
They may not necessarily be actors in adult videos.