This is a fun fantasy for me. I'm one of the M's in the MMF threesome. But it would be a bit twisted. I'm 44, my gf is 35, and we love to play with power dynamics. I'm def the dom in the relationship. She can be dominant in her friendships and with other people in social situations but sexually she is very much submissive to me. She's tried to usurp my power and control and it has always ended poorly (read: with great fun) for her.
My MMF threesome would be the two of us, me and her, answering the door for a chubby, nerdy, socially awkward, uncomfortable 18 year old boy. He is out of shape, has no friends, is overweight and has zero prospects of girlfriends. He's definitely straight but he couldn't know because he's untouchable.
My gf seduces him. He comes to our door, maybe doing a canned food drive or collecting money for Habitat for Humanity. She invites him in, listens to him, and I lock the door behind them. She takes off her clothes, not making a big deal out of it, and strips nude in front of him. I'm already hard just typing that now, thinking about her exposing herself, exhibiting herself for this kid. He doesn't know what to do, just stops, just freezes.
I take my turn first. I kneel in front of him and ease his cargo shorts off his hips. He is confused, panicked, but there's a beautiful, curvy nude woman in front of him and he's horny, scared, and uncertain. We show him the way.
He has a small penis, not even 4 inches, and it's sticking out from under his big belly. He has a sparse patch of hair and I kneel in front of him. My gf guides his hands to her tits so he can touch them and she holds his belly up while I suck his cock. I keep him hard, get his cock wet with my spit and then my gf gently pushes him to the ground on his back. She mounts him easily, sliding onto his cock, and starts fucking him, riding him. I stand so she can suck me off while I watch her.
The boy is going crazy, barely lasting and it doesn't take long for him to cum. He unloads into her pussy, screaming, crying, sobbing at the experience and the relief. She slides off him and cleans him up with her mouth. We kick him out of the house without any more words, and she has taken yet another virginity.