An elaborate thought-code, self-administered in a trance-like state (or simply a consistent desire, even if not conscious) will lead, over the course of about a year, to what amounts to a viral change from one sex into the other. The convention - tradition, even - in the Culture during the time of the stories written so far is that each person should give birth to one child in their lives.
In Excession there's a male-female couple who wanted to be mothers to each other's children. So the woman becomes pregnant, both parties switch sexes (the woman storing the undeveloped zygote in their now-male body for the time), the other now-female partner becomes pregnant, then the now-male switches back to female so they could both gestate their children together. The Culture is even more FALGSC than Star Trek is, I swear.
And when they don't, like Gurgeh in Player of Games, it's specifically pointed out as an eccentricity on their part. He gets lightly teased for being cis.
Edit: found the quote
'There's something very I don't know; primitive, perhaps, about you, Gurgeh. You've never changed sex, have you?' He shook his head. 'Or slept with a man?' Another shake. 'I thought so ,' Yay said. 'You're strange, Gurgeh.' She drained her glass.
From Iain M. Banks' A Few Notes on the Culture
In Excession there's a male-female couple who wanted to be mothers to each other's children. So the woman becomes pregnant, both parties switch sexes (the woman storing the undeveloped zygote in their now-male body for the time), the other now-female partner becomes pregnant, then the now-male switches back to female so they could both gestate their children together. The Culture is even more FALGSC than Star Trek is, I swear.
The Culture is a lot more FALGSC than the Federation
And Excession isn't even the only Culture novel where the main protagonist changes sex during the story.
And when they don't, like Gurgeh in Player of Games, it's specifically pointed out as an eccentricity on their part. He gets lightly teased for being cis.
Edit: found the quote