Disclaimer: this is a fantasy written by an adult for adults, The author doesn't condone rape nor incest in any situation besides role-play between consenting adults.
I once heard a statistic that something like 1/10 adult women have never had an orgasm. And that seemed kinda sad to me but it did give me some inspiration for fantasies about giving a woman her first orgasm. That'd be nice and intimate. I combined this idea with thinking about how hot it would be if the first orgasm a woman had was from her own son because I'm a perv. Frankly I would say that
So here's the scenario I'm thinking of, a single mom has lived her whole life never having climaxed. She's been hearing her son having sex with his girlfriend/girlfriends and she gets jealous, she's never actually had a real orgasm before. So she walks up to her son and asks if he could give her some pointers on how to masturbate to completion. She's been really repressed her whole life and kind of needs some help.
The son in this scenario is really interested in not only fucking his mother, but knocking her up and turning her into his pregnant little mom-wife. So he hatches a plan, insisting that he can't just give her pointers but has to be there to see what works. From there the actress is basically describing what she'd normally do to masturbate, but acting as though someone is telling her how to do it.
After her son teaches her how to masturbate a little, he tells her that she's "doing it wrong," so he does it himself. She protests because a son shouldn't be touching his mother like that. But she tries to convince herself that it's only for education as he continues to molest her. He continues to touch her until he tells her that she needs to be fucked in order to have a real climax. She tries to draw the line there but he teases her with his cock until she agrees to only have it in her until she orgasms.
He then proceeds to fuck her with the enthusiasm she deserves. He tells her he's going to cum and she tells him she's ovulating, begging him to pull out. But he cums inside her while she has the first climax of her life.
She's freaking ecstatic, having had her first orgasm. She's not to worried about pregnancy because she's planning to take a morning after pill... until her son pulls the rug out from under her. He doesn't threaten to blackmail her, but says that if she doesn't keep the baby, he'll never have sex with her again. She then gets outraged, you can't just show someone a new world and then never let them experience it again. She then agrees that she'll keep the baby so long as her son keeps fucking her.
And they all lived happily ever after.
While it's not required that the actress have a real orgasm as opposed to faking it, I'd much prefer it, partially because it makes sense for the character but mostly because the actress would deserve to feel good. One should never under-appreciate the kindness of a horny stranger.