Not going to use a throwaway, but I'll likely delete this in a few days. Warning: it gets graphic.
We played world of warcraft pretty heavily. Our marriage was rocky from the start since we got married young for religious reasons rather than a true connection, so when things were particularly low she would form sexual relationships with guys in game after I went to bed.
One such guy was a friend of both of us and an officer in the guild we were running. He's overall an okay guy, but they consorted to meet in real life to make some fantasies happen. I didn't know this, of course. They didn't know how to make it happen since he wasn't exactly local and I would want to hang out with him, too, if he visited, so she came up with the idea of convincing me to including him in our bedroom activities.
When she brought it up to me, I didn't think she was serious. I thought it was some game she was using to spice up the night since she was always interested in talking and telling sexual stories during sex, so I agreed and had a fun night with her.
The next day his flight came in. I took the day off to pick him up from the airport and stuff, and then we picked her up from work. As soon as we got into the car she was visibly excited. She said to him, "there's something we'd like to ask you." My stomach dropped to my feet and I started shaking, and when he agreed it got so much worse. I tried to back pedal out of it, which worked for the most part. I said I didn't know how serious the conversation was and wasn't sure how comfortable I was with it all. I still tried to leave it open, though, because I was in love with her and it was obviously something she wanted to try.
We went out to dinner, then to a sex shop to try to boost my confidence. He drove the car while she and I played in the back seat. When we got home her clothes started coming off while kissed her and played with her and he sat on the couch watching. I got the nerve up to have him come over and pull his pants down. She started sucking on him while he fondled her breasts and I rubbed her clit.
She then got onto all fours and sucked him off while I tried to enter her from behind, but she didn't angle for me and I started pushing against her asshole. She spoke up saying "your doing that already?" all irritated sounding, which instantly made me flaccid.
We tried to bring me back by changing her positioning with me eating her out while she stayed focused on his dick, but I was just feeling more and more embarrassed and eventually took her upstairs to be with her in bed alone, which made it pop back up just fine.
So we were in bed and I'm taking her slowly and steadily, but she wants him in there. She kept pushing it, which was oddly sexy in its own right. She offered various sexual favors, including anal. We even started the butt stuff to try to get me going, but I just couldn't be convinced to bring him in.
The next morning we all hung out and talked a little about it but nothing big. He didn't try anything and neither did she. When I went off and took a shower and she snuck in and blew him, though.
We're divorced now. Not because of this story in itself, but we were not doing well enough in our marriage to handle something like this.
TL;DR: Tried to include a guy with my wife and I, turned out it was based in planned infidelities.