So I've got this mental collection of imaginary boyfriends that I can call up and then either pick up where I left off or go back and re-edit it again. It's also something for me to focus on as I am drifting off to sleep, especially if whatever else is on my mind is stressing me out. I allow myself to imagine very specific details about the whole encounter, not just the sex although that serves the immediate need, but also what a relationship might have been like and how that would have let me down a different path than the one I have walked.
I sort of do this, except I do a lot more than imagine scenarios. I go so far as to make up back stories, conflicts, and entire plots. One of mine occurs in an alternate reality, because this one wasn't interesting enough for me, I guess. I started writing them down when I realized how complex some of my stories had become. After I had written quite a lot, I finally confessed to TheBF what I had been doing and after much hand wringing, I finally let him read just one little chapter. He loved it so much I let him read a little more. He thinks I should publish it, but I don't know... it feels too personal to be something to open up to criticism.
I don't consider that to really be imaginary friend territory. I think that's just being really horny but having an active imagination, too.