I've seen several porn movies where there is a couple having sex and seated a few feet away is a single naked guy masturbating as he watches them. I really don't understand the appeal of a scene like this. I would think the last thing straight men watching straight porn would want to see is a dude stroking off in the porn.

Any insights as to why adult movie makers put scenes like this in their movies?

And why the hell would a guy agree to get naked and jack off in front of the camera and not even be in on the action he is watching? It can't be the money, they don't get paid much. It seems to me most straight guys would be humiliated to be seen by thousands of viewers jacking off while watching a sex xcene. Don't they worry about friends, family, and acquaintances finding out? Thanks for your insights and opinions guys.