I think this is actually one of the more clever points Banks makes, although not explicitly.
Fundamentally, the Culture believes that living things (and their definition in this regard is remarkably broad) have a moral right to exist. Therefore, as a society they are not expansionist. In order to remain non-expansionist, the population must be kept stable and this has implications either in childbearing or lifespans. The average Culture human mothers about one child but that means they can't, on average live forever. Why they choose to have children at all perhaps also boils down to the future generation's moral right to exist, but also because they recognize that a renewing population means a renewing culture and Culture.
In this light, I believe it's easy to see immortality as a sort of childish self-aggrandizement comparable to wanting to become the ruler of some backwards planet. Skaffen-Amtiscaw (an artificial entity and citizen of the Culture) even remarks on Zakalwe's immortality as childish in Use of Weapons.
The Culture never appeals to nature – how could they, they are ruled by their Minds!
(Mind is a sort of very powerful artificial intelligence).
There is a lot of similarities between the Culture and Trek, they are both visions of post-scarcity humanity made impossible by the simple fact that humans could never be that nice.
A little while ago I met someone whose job is to worry about international affairs and they were worried about operating systems.
After that I started worrying about every end user device in Ukraine shutting down until peace was negotiated.