I lost hope a long time ago of finding this story, but I'm tossing a line out there to see if anyone can help.
I have to imagine that when I had this story on my hard drive it was around the year 2000-2001. It might be slightly older or slightly newer but it can't be anywhere recent.
I don't remember what site I read it on originally, I browsed a few different sites for stories at the time, the nifty archive, asstr/kristen archives, whiteshadow, and of course, this wonderful site! I don't think it was a story written exclusively for one site so my thought is it might have been submitted on multiple sites.
Anyway, the story as I can remember it starts out as narrated by the "Father." It is a nice story, meaning no non-consensual sex, no rough stuff. I believe I remember the family structure as being the narrator (Father) and his son... married to the wife and her daughter. The father manages to see his step-daughter masturbating through a crack in the door to her room... he stops to watch and then hears a voice in her room. He at first thinks this is some random kid and is about to enter the room but then sees that it's his son. He orgasms in his pants and hears his wife come home.
The story becomes hazy in memory at this point but eventually they all have a group thing happen where the father is having sex with the daughter and the son is having sex with the mother. They are teaching the kids and at one point the mother is talking about the father having a "golden tongue" or perhaps a "silver tongue," I'm not sure which. I also remember a scene between just the parents where they are having sex and pretend to be the kids.
I want to say I remember the wife as having the name "Jen," possibly being the narrator's nickname for Jennifer. The son I think had the name Mike. I cannot think of the other names for the life of me.
It was a one part story as I remember it, but I have a feeling like I found a second part to it at some point.
It's extremely frustrating to not be able to find this story as it was one of the best I ever read. Very well written and sexy.
I wish I could remember the name, the author, something that would help me find it or help describe it to other people... but I hope that someone will recognize this.
I'll end this desperate plea by saying that I have been searching for years now, using various Google techniques and every website's search function. I've used the wayback machine. I have tried everything I can think of.