This is an excerpt from one of kinkybelle_bi stories titled "Seducing Our Son - Chapter 3" at Literotica. These passages did it for me. It's quite a unique story, with the mom character planning for her husband to suck his son's cock. You don't get to see such plots in a mom/son story every day.
I was making my fantasy come true. I was going to see my beloved husband suck his own son's cock. It was one of the best sexual gifts a mother could wish for.
I did my best to make it an easy transition for them. I wrapped my lips around Evan's cock once again, and gave him a few strokes with my mouth. I took it out and directed my son's cock toward my husband's face.
"You guys look so sexy. I like seeing my husband with a big cock in his mouth."
Don was getting the hang of it and his head was bobbing a little faster. He surprised me by reaching up and massaging Evan's balls as he blew him. Evan then reached out and put his hands on his father's head, just like he had done with me.
"That's it, baby," I groaned. "Fuck your Daddy's mouth right in front of me. Make him suck your cock real good."
I was pinching and pulling at my nipples, upset now that I wore the body suit as it prevented me from giving them the harsh treatment I craved at this moment. I did the best I could as I continued masturbating my pussy like crazy.
"Oh, yes," Evan moaned. "Don't stop, Dad."
I knew this was it. My son was about to cum in my husband's mouth. Don wasn't pulling away, he was actually going to go through with it.
Evan's body tensed, and his face contorted, and I could tell he was squirting his load in his father's mouth. My son was actually cumming in my husband's mouth.
The Fanstasy: Picture a slim, hairless Leonardo De Caprio thrusting his long thin cock into the waiting mouth of Alec Baldwin (where did that come from ?!) and splashing the back of his throat with thick ropes of sweet, frothy cum... as the mom, Kim Basinger lies besides them, stroking the blonde curls of her thick, matted pussy.