(Simple fantasy story, doesn't bother with logistics or world-building. Set in a world where freeuse is universal, mutual, and consequence free. Fill in the blanks as you see fit.)
Jackie came home from work tired after a long day at the diner. Her son, Bobby, was already home, leaning out on the sofa chair watching TV. He was in his paint-stained work shirt and his boxer-briefs, his boots and pants kicked off right near where he sat.
"You're home early," Jackie said.
She let down her hair, past her hoop earrings to her shoulders, very tightly curled and brown enough to almost be black.
"Hm? Oh yeah. A bit. Should pick up more next week," Bobby said, lifting his head and orienting himself. He looked more like his father: tall while Jackie was short; a ruddy freckled complexion while Jackie's skin was a pale white ivory with the slightest touch of olive. But Jackie and her son shared their long faces and aquiline noses.
They chat as she took off her shoes and her dress. She came over and kissed him on the cheek and turned around so he could unclasp her bra, so she didn't have to reach around and fumble with it. She sighed in relief as the bra came off. She'd gained some weight recently: not much, and she'd been skinny her whole life, so it didn't bother her that on the far side of forty she'd started putting on some pounds. But she would need a new set of bras. And the weight going to her butt might be why her panties kept riding up and giving her wedgies more often, she thought, as she slid her panties down and curled up next to her son in the sofa chair.
Bobby made some small talk with her about her day and let her complain about her coworkers as he licked on her nipples. Then he talked about his new job painting while Jackie slipped his dick from the waistband of his underwear, turned a bit and nestled her butt into her son's lap so his penis slipped into her pussy. He started grinding into her, slowly so the chair didn't rock too much. They went like that for five minutes, and Jackie was surprised because it rarely took her son this long to cum once he was inside her.