This is the problem.
I know you were acting in good faith, and 10 years ago we were living in a different world, but this is a real concern now.
Every time a woman calls someone out and gets it wrong, it gives those who have are using the space when they shouldn't be, the ammunition to argue that we can't always tell.
I don't think I could call someone out (unless I was 100% sure) but I would probably chose to leave the space, perhaps even with a comment that I'll come back at a more suitable time.
But we absolutely can't be misgendering woman. It's really not fair, and a female should be able to present however they wish.