So I have to move my entire extended family to an orbital? What if their demands aren't met there? You're missing the point about spatial scarcity and demand.
I'm guessing you've not read the series, because the context is seemingly entirely lost on you. Culture citizens are free to go anywhere they want and do literally whatever they want. Small orbitals are ~20x the size of earth whereas large ones are 200x the size of earth. Also, you can live on planets if you wish, but the idea is seen as very quaint and generally not a very "culture" thing to do.
When you can move literally anywhere in the galaxy, amongst virtually any galaxy that hyperspace ships can take you to, there isn't a scarcity of space and machines can create literally anything in this world on demand, so demand is also never a problem.
I think somehow we are misunderstanding eachother, but it makes it much harder to explain this VAST ecosystem when it does appear you might not have even read the series we're all discussing.
It's not dwelt on much, but to make this work requires restricting production of humans. The fictional mechanism seems to be a universal sense in the Culture that reproducing at more than mildly above the replacement rate is gauche and atavistic. Presumably it's nudges from the Minds keeping us collectively in bounds.
The 20 richest countries[1] in the world are already below replacement total fertility rate. Most of the top 50 are. And even now, in a rich country like the US, about half of pregnancies are unplanned. Birth rates would drop a lot if pregnancies were only possible when both partners consciously opt in.
Obviously the replacement fertility rate is going to be a lot lower in a society where people can live as long as they please and most choose centuries of life. But the Culture is also vastly wealthier than any real-life low-fertility nation. There's perfect birth control for both sexes and a general absence of destructive impulses among biological Culture citizens (due either to selection pressures or editing). I'd guess that the Minds don't need to nudge much if at all.
Having advanced that argument, I wonder if there is any consistent relationship between material prosperity and fertility. Would a weakly post-scarcity society[2] have higher or lower fertility rates than rich nations do today? There'd be none of the security-in-old-age pressures that drive high fertility in poor nations, but also none of the economic penalties to child rearing that discourage high fertility among the educated middle class in current developed economies.
[1] Ranked by nominal GDP per capita.
[2] A fully robotized productive economy, but we can't just gin up atoms and energy like in the fictional Culture. Also no FTL or teleportation. Real physics only. It's the Silver Plan of post-scarcity, as compared to the Culture's Unobtainium Plan.
There are sects in rich countries with fertility many times replacement. They're easy to overlook because they started tiny and it's only a few generations after the demographic transition. Lower birthrates in the general population only shorten the time to the new growth mode.
The Culture is not limited to biological rates of reproduction. That nobody ever sets off a human forkbomb is a collective choice, somehow.
