@limnophile75044Thanks everyone, for the insights and advice.
I'm thinking I'll make the young woman MC and her great-great-grandfather part of a much larger sci-fi adventure story. G-G-gramps is one of the first 'immortals', in love with her but morally conflicted when they discover their relation. Younger people have adapted to incest being normal and most are fine with it. As what seems like a minor bit of background, they hear news that a 'super-do-hickey' (haven't decided on type of device - tiny and nearly free power source? wormhole generator? ultimate weapon?) has been invented and a technician disappeared with the prototype and only copy of the plans. Eventually G-G-gramps decides he can't stand the guilt anymore and leaves her for another hottie he's not related to. (MC looks 21 and G-G-Gramps looks 23 if that matters.)
She finds a new boyfriend who was practically a sex slave to his widowed mother until he ran away. His submissive personality allows her to get out the pain and anger at being abandoned by spanking him, etc. They have a chance encounter with a hot couple who happen to be the 'super-do-hickey' thief and her husband/uncle. The group flee from world to world trying to find a safe way to sell it as they experience many strange settings and societies. In the year 4,500 humanity would have spread to thousands of other planets. On worlds that are densely populated people are only allowed to have one child, plus an extra one or two as rewards if they do something great. On freshly settled planets, they can have as many as they like. Some small new societies would even give a reward for having lots of kids.
In the setting I'm imagining people are physically mature at about 18-19 like the real world. They are reversibly sterilized and given sexual desire inhibitors as young children, and at 20 they get a treatment which greatly slows aging. They start looking 30-ish at 400 years old and gradually lose their ability to reproduce once middle-age sets in around 800. Many decide they've lived long enough and agree to euthanasia before reaching maximum longevity at 1,200.
On most worlds, parents have custody of their children until they are 40 and teach them sex from age 25 to 40. A special area of each community is reserved for schools, universities, and families with children. For safety's sake, only people 25 or older are allowed to leave the 'nest' neighborhood. Regaining fertility only requires a short doctor visit once they reach 40.
The idea that people aren't allowed to even think about sex until they are 25 or marry before they are 40 is based on the rules of some restrictive websites, not SOL. With anybody under 25 safely confined to school or home, the rest of the world can do whatever they want, wherever they want. This frees up the possibility of outdoor orgies in public parks, horny couples screwing leaned against a building downtown or on city transit, nude bicycle or foot races, advertising billboards with product names around a screen that plays porn... pick your favorite Voyeur/Exhib fantasy.
It also makes 'age room' to include a planet where a rebellious religious group arranges marriages for their children at the shockingly young and illegal age of only 21.
I expect it to be a very long story, probably 100k words or more, so I doubt I'll publish it next month. LOL
Thanks again!
Any other advice / suggestions?