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.
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