I can contribute a story:
Just a couple of months ago, my partner (f) and I (m) had an old friend visit from out of town. We've had threesomes before with this friend, and she consistently says he's the best fuck ever. Him and I had threesomes with other girls before and all of them say the same thing. Seriously, this guy must do magic with that cock. Now, I'm not gonna lie and say he's bigger than me, he's not. But she always tells me his cock is harder, and he fucks her better than I ever could. Still, he never knew what my partner thought of him.
When he came out to visit, she had been flirting pretty hard with him the whole week, but after two nights we hadn't done anything. Finally, her and I are gonna fuck one night and she tells me to hold on just before she starts. She goes into the other room, and I'm still waiting. After about five minutes, she comes pulling him by the hand into our room. He's hard as a rock. So I'm thinking it's gonna be another threesome, and for the most part it is.
She lets me fuck her while she blows him. She's going to town on his dick. then she switches. However, something changes while he fucks her. After about half a minute, she says, "oh, fuck," and just completely forgets about me. She cannot stop moaning. He loves it. He's just going to town, thrusting and thrusting. I keep trying to motion to her to blow me, but she does not care. Eventually, we switch again and she gets on top, she is seriously wet. I look over, and would you look at that...she squirted! I've never mad her do that.
So she's riding me, and I see from underneath her blowing him while he's standing. Again she's getting down on his cock. I don't cum, but after about five minutes I take a restroom break to piss. And fuck, i hear her moaning again. I clean off and come back out, and she's on top of him. I sit back for like two minutes to watch, then he gets up and goes to the restroom himself. On his way out, he says, "she's all yours." When he leaves she motions for me to fuck her. She gets in doggy, and looks back to say, "he came in me." I think she's joking, because she knows I'd like it. I go to out my dick in, and there it goes, pouring out. A lot. She backs up to my dick, and I put it in and fuck her even though she's filled with my best friend's huge, warm load.
He comes back in, and seriously?! This guy is still fucking hard. I cum in less than five minutes. I finish in her. He looks at me and asks if I'm "gonna finish that?" I look at him and he points to her pussy. I'm shocked, but I guess she told him about my fetish. So I do it. He tells me to make sure to get it all out. He's stroking this whole time. I clean her out then go to the restroom. I tell them that they can keep going if they want and they just laugh.
So, I'm in the restroom cleaning off, and there they go again, she's moaning and saying his name. I come out and he's fucking her doggy Style super hard. Now, I can never cum and go immediately again, but he can and he's fucking like he hadn't already just ten minutes before. I watch for a bit, but then I go to the other room. I'd say they fucked for a good 40 more minutes after I left (and about 24 before). When he comes out, I'm sitting down gaming in the living room. I sits down and I ask him if she told him I'm into the whole hot wife/cuckold thing (i know it's not the same thing...but it's what i ask). He looks at me and says no, but was just "feeling extra kinky earlier."
I come to find out later that she had told him before over text that he fucks her wayyy better than me, so I guess he just picked up on it. Still, i learned two things: 1, my girlfriend is no longer just mine (they fucked almost every free moment the rest of that week); and 2, im now a cuck in the eyes of my best friend.