Catching stepson masturbating leads to JOI and mutual masturbation

The other day I walked in on my stepson looking at porn on his computer and masturbating. I should have knocked but I didn't. I start to back quietly out of the room before he can see me. However, I can't take my eyes off of him stroking his cock. I had no idea it was so big. I know how wrong it is but I can't help but be extremely aroused by the sight before me. Before I know it I am touching myself. I close the door and go to my bedroom. Once in my bedroom I can't hold back. I pull my shorts off and begin to masturbate thinking about watching him jerk off. I vow to knock next time. But of course a few days later I walk right in again and witness the same scene, him jacking off to porn. Again I am so turned on I touch myself then go straight to my room to masturbate. I realize I need to talk to my stepson so I confront him and tell him what I saw and what I did. He is first shocked and embarrassed but that quickly turns to arousal. I see the bulge in his pants growing...