My gf [40f] asked me if i would watch her with her coworker

She said it would spice things up as admittedly things had gotten a bit stale in the bedroom. Ive never been an adonis or anything and im definitely not killing it in the sack so i figured if it would get her sucking my dick again then so be it. She assured me it was just sex as he was too young to be a threat (hes 23) apparently he just talks avout how much he wants her all the time and she fantasizes about it. We started off just with some roleplay. I blindfolded her and we pretended i was him and WOW. She was feral. Left grooves in my back and everything. I felt like such a stud. After that i naturally agreed and she happily went to work the next day to try and set it into motion. It wasnt an hour into the shift and she said he was down and once again reassured me that it was just sex. Fast forward to the weekend when we can find a sitter for the kids. Who was appropriately the same age as her soon to be lover! We send them off and shes spent all day cleaning and getting ready and she looked radiant soft and fuckable! Me and her plan was i was going to sit in the livingroom while she invited him in and brought him to the bedroom where they would warm up and i would come in when they had some rhythm. He got there and insisted he wanted to talk with me to make sure i wasnt going to kill him! (Im 6'4" and 250 and he was 5'10" and probably 175) after a short chat he said we should go to the bedroom unless we wanted to start in the livingroom. 73 people can see our livingroom in our building so to the private bedroom we went. Im not going to build suspense. He massacred her. They were naked in a flash and he had a bigger flaccid cock than i do hard and he guided (lets call her natasha) head towards it and i watched entranced watching. Ive never thought of myself as a cuck and was having some very conflicted feelings watching his cock getting larger and thicker. He was probably 8". I know there larger but he was thick like a coke can and was embarrassingly larger than mine. As soon as he was hard he went down on her and wirhin a couple minutes he made her cum, hard, before she could stop panting he pushed her face first into the bed and said being caring was my job. He took her from behind. He obviously was used to this routine and let her get used to his length and girth slowly but she already came for the second time. She told me it was purely mental from knowing im watching her "get split in two" and he picks up the pace slowly again. She was so responsive i couldnt believe it and it made me so insecure. Im lucky if she cums twice in a week with me and this kids done it twice in less than 10 minutes. After that it was all a blur. He pretzeled her up, was very vocal in telling her what to do and kept bringing me in for a close up view. He liked seeing my face near her ass. Well he beat her up for another 30 minutes and he pulled out. Blew a huge load on her face and basically immediately got dressed and she was laying in bed sweaty and redfaced and shaking. He left, we had sex. It felt like a joke compared to what she was just out through. She wants to see him again. Im not sure