This is my story, also published elsewhere.
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It actually started with him telling me that he'd slept with guys in the past and that he'd like to do it again. As he'd only seen him act dominant sexually, I really liked the idea of seeing him submit to another man. Initially, he was uncomfortable with me being there in person, but as I got to know the guy he was messing around with (A handsome, closeted bi man in his mid-fifties) I started receiving pics of my boyfriend sucking his cock or bent over a couch, cheeks spread, asshole open from being freshly fucked. This, I later learned, was my boyfriend's idea, even though they were from the man's phone. Then, as I was home alone one night my bf claimed to be working, I got a text with a website, username, and password. I logged on and a live stream from a webcam appeared. The bedroom it featured was only empty for a second before I saw my boyfriend walk in from a door across the room, naked and wet, he hopped onto the bed.
The man entered with a towel wrapped around his waist and as he passed the camera, he threw it off, revealing a flaccid penis bigger than any I'd seen in real life. I could tell he'd known I'd be impressed with his cock by the way he smiled into the camera. As he approached the bed, my boyfriend eagerly crawled towards him, opened his mouth and looked up at him with doe- eyes as the man fed his cock to my boyfriend. A few minutes later the man looked at the camera, winked, and like he weighed nothing, flipped my boyfriend onto his back and around so his ass hung off the bed. Finally, I could see the man's hard cock unobstructed. It was bigger than I'd imagined it would get, with its foreskin pulled back exposing a head like an apple. I could only see it for a second before he grabbed it with one hand, pushing my bf's legs above his head with the other, and expertly fitting it into my boyfriend's asshole.
Later that night I got a text from the man saying "Hope you liked the show, no need for ... to know". This started us chatting behind my bf's back. Mostly about the two of them fucking and whatever. Each time he'd text, I'd picture his massive cock. After about a month, I got a picture message from him. It was of his soft cock. The message read "need some help here". I couldn't believe it, we'd never talked about anything sexual that didn't involve my boyfriend. I was at work at the time so I went to the bathroom and sent him an upskirt. Before the message was completely uploaded and sent he texted "whoops meant to send that to your bf". It was too late and I didn't care. We agreed to meet at his place so I told my boss I felt sick and left. I took a cab and was at his house in ten minutes. Immediately after he let me through his front door, he asks me to strip. Which I gladly did. I stood there, bare and blushing as he inspected me like you would a car. He squeezed and palmed my boobs and ass firmly. He then stuck a finger in my mouth to wet it, before putting it in my pussy. I was so wet it slid in easily. So there I was, naked in the doorway this man's house as he fingers me, and I hear a creak behind me.