Noun: A guy who always has a harmonica on him. Unlike guitar guy, he does not have the harmonica on him specifically to impress others. He has it on him as a security blanket, to practice wherever he wants, to busk for money, and sometimes to impress others.
Basically Harmonica sex is where the girl (or guy) typically a bottom is handed a harmonica, they keep it in their mouth the whole time so that any sex noise is translated into a harmonica noise.