How would watching jerk off be hotter than getting cucked?
You get to see him in action naked dominating the love of your life and treating her like a whore
Your gf will probably have a better experience than she normally has with you
He will humiliate you verbally and maybe even physically
You get watch his cute lil booty and hot powerful body Pump your gf
You may even have a chance to serve him by bringing him refreshments or massaging/kiss his feet/muscles in between rounds
Plus it feels natural to submit to more alpha men like hook