I’ll try to explain a bit more where I’m coming from.
[SNIP]
I also have a colleague who is a trans man. He looks and sounds like a man, and I suspect many people at work who don’t know he’s trans assume he’s a cis man. If I believed that the words ‘man’ and ‘woman’ relate to sex rather than gender, and that a person with xx chromosomes can never be a man, I would believe he was wrong and that he was in fact a woman. However, I don’t see why my personal beliefs about sex and gender should take precedence over another person’s, when it comes to their own identity and how they are referred to, any more than my beliefs about what it means to be gay/straight/bisexual. So in the sphere of gender and sexuality, I refer to people in the way that reflects their identity.
We already know where you're coming from. You have an end outcome you are aiming at, and you are coming up with reasoning to get there. Telling us about the work colleague doesn't change what you are doing.
Now let's go for a topic a little less fraught.
The word vegetarian means someone who does not consume meat, poultry or fish. Some people call themselves "vegetarian" when they do eat fish. What you are arguing is that their beliefs should take precedence over mine inside my own head!
What I normally do in real life and the workplace, is simply ignore people eating fish while calling themselves vegetarians, unless they explicitly ask in some way for me to agree that they are vegetarians, or start trampling over my rights. The answer is no. I will not lie for them or you.
For example, if they use their identity as vegetarians to push for the subsidised work canteen to replace the current small selection of vegetarian options with seafood dishes, they will have pushed me to the discourtesy of telling them they are not vegetarians. I will need to do so in order to express my own needs. They will say it is rude of me to do so. I think it is rude to push for a menu change that will mean I and all other actual vegetatians will be excluded from the subsidised canteen!
They will hear statements they do not like from us actual vegetarians, and the fault for this will lie with them, the fish-eating pescetarians.
You cannot force me or expect me to see you as vegetarian, or anyone as a woman or a man when they are not. The best you can do is either fully deceive me or accept the social contract that everyone tolerates everyone else having different thoughts. I will keep to my side of the social contract as long no-one starts imposing their beliefs on me.
Another example might be Islam. I believe women who wear hijab shouldn't be discriminated against or excluded from public spaces, either directly or indirectly (note that many trans activists pursue campaigns that will indirectly exclude women of visible faith). However I don't believe any woman should feel obliged to cover her hair. I do not voice this to women who wear hijab because I think it is rude to do so. However, if you try to impose your beliefs on me and push me to agree that women should cover their hair, I will be loudly telling you about my beliefs. This is called the consequences of your own actions.
Again, no-one has the right to make me see them as the other sex than they are, either.