A strap on is a cast of a penis that is then strapped onto a person to recreate the act of getting fucked by a guy. Who is wearing the strap on is irrelevant, the fantasy is centered around getting fucked like a girl by a guy. He's gay.
Quoted for truth...you can't explain it any clearer than that.
Hmmm...I wonder if the gay biz will start making a series of films where guys wear strap-on pussies and get fucked like chicks?