Here is a taboo roleplay prompt with setting, scenario, and characters sketched out. I would like to find a woman to finish writing the story with. You could write it from either the perspective of the girlfriend or the mother.

Scenario:

When my girlfriend told me during breakfast that her mother had started an Only Fans page, she intended the comment as an exasperated joke. "Can you believe how far an older woman will go for male attention?!?" I was astonished and laughed with her, but the thought grabbed me in a way I don't think she intended. I had always secretly regarded her mother as very attractive. She was too old for me to date, but more than attractive enough to steal furtive glances at, and the idea of her succumbing to a primal need for sexual attention immediately sent blood flowing to my cock. I laughed it off with my girlfriend, but all day the thought circulated through my mind, and when my girlfriend stepped out to see her friend for dinner, I immediately did what I suspected I would do: I opened up every Only Fans search engine I could find and began browsing for her mother's page. I could join anonymously; jerk off to her pictures; maybe even dirty talk with my girlfriend's mom! The possibility was incredible.

But unfortunately, finding her page turned out to be much more difficult than I thought. I spent weeks looking, but it was only when my girlfriend and I visited her house for lunch that I was able to find out what I was looking for. By then I had a crude plan: excuse myself to fix a few details in a short report for work ("yes, I know working on the weekend again, but my boss is a perfectionist!"), leave my laptop unplugged so it was low on batteries, "forget" my charger, and ask if her mom would let me use her computer. "Does your mom have a computer I could use? It won't take long?" Of course, she did, and it took me even less time than I pretended to find what I was really looking for: the name of her Only Fans. She wasn't even logged out. I even developed a mnemonic so I could safely commit it to memory.

That evening, after my girlfriend was asleep, I searched for her mom's Only Fans, and this time I found it without a problem. I joined with a prepaid credit card I had bought a week earlier at the drug store, so it wouldn't show up on our shared card or my bank statement.

At first, I told myself that I was just getting her mom out of my system, but it had the opposite effect. I found her mom's desire for male attention to be very real, and the more daring I got with my comments to her, the more shocking became her replies. I'm sure that I was pushing her thoughts in a direction she had never before considered, but she surprised me with her willingness to get filthy...

As her mom became my main taboo fantasy, I found myself guiltily wondering how I could get my girlfriend back into my fantasy system ...hopefully, with her mom in tow! "Honey, can we dirty talk tonight?" I innocently asked her that evening. Then, "I have an idea: How about we exchange fantasies each week? The naughty ones. I'll go first, then you..."

After a few months, my bank account was missing far more money than it should be--I suppose that if my girlfriend ever looked at my account carefully, she might suspect that I had developed a drug or gambling habit. Meanwhile, my girlfriend had revealed herself to be much naughtier than I had ever expected her to be, and I was running out of generic adolescent male fantasies to pretend I was into ("Honey, how about you dress up like a Catholic school girl?"), and I had mentally fucked all of her cute girlfriends with her. This week was my fantasy night, and I knew exactly the piece of filth I wanted to spring on her.