I actually misread your comment at first as saying you thought of
not submliming that way, and agreed.
To me subliming seems like trying something new that also frees you from the restrictions of a normal physical existence, where the Culture seems to feel tied to their bodies and to a kind of existence that is ultimately limited by that physicality.
But the Culture is conservative on average even within the constraints of the Real - while we hear of "outliers", ultimately the Culture has a lot of norms that people seem to rarely stray outside.
A society where people constrain their lifespans out of choice would seem to be one where people feel they run out of things to do, but yet they seem to be only scratching the surface of the possibilities their material abundance creates. In Excession there is musings about the feasibility of a trip to Andromeda, for example, but you'd expect a society full of people voluntarily dying to have more people prepared to set out on ridiculous voyages or doing other ridiculous things just because they can.
That so few do things like the person who had a vast number of penises added to his body suggests that while they are open minded enough to let others do as they please, they are themselves pretty conservative. I mean, I don't expect that specific thing to be for many people, but I'd expect an infinite number of possible variations people might want to try. I was re-reading Transmetropolitan last night, and for all the other things that are now hilariously dated in it, one of the fascinating part of it is gene-traits as fashion. As much as I love the Culture, Banks seems to have stopped short, finding extensive drug-taking and gender changes to be radically crazy enough for at first (it's notable that the penis-man came in The Hydrogen Sonata - the last Culture novel). But then people get enough of living.
Of course, perhaps their relatively conservative approach is in part because the more adventurous ones leave. We do have a hint of that: The Zetetic Elench is a former Culture faction that left to pursue change and knowledge where the Culture wanted to stay as it was. I guess we can assume that the Culture regularly have people leaving for the Ulterior or leaving entirely if they feel too adventurous to fit into "regular" Culture norms.
And I guess that shouldn't be surprising - in a truly free society of a sufficient scale with enough abundance for people to just up and leave on a whim, it might not be surprising if clusters of more conservative civilisations develop around those who have found the most effective ways of maintaining cultural cohesion.