My girlfriend's legs were spread open and she was being penetrated by a male PE student. I wanted to see her cunt being filled, but I...

My girlfriend's legs were spread open and she was being penetrated by a male PE student. I wanted to see her cunt being filled, but I couldn't. I could only perform footjob on her from behind, watching her feet sway wildly in rhythm with his penetration. I unconsciously masturbated to arouse them. My girlfriend was so turned on that she was swinging her legs, wanting to take his big cock even deeper.