Maybe they're better placed to judge what they are or are not than you are? Equally it could be asked if someone requests that you refer to them in a particular way then what is the harm in going along with their wishes?
This subject is a minefield recently. Seeing it more and more in the work place.
Now I'm pretty relaxed and open minded with these things. If someone wants to be referred to as they, their, them or any other unisex or sexless term that's fine by me and I will respect their wishes.
However the big issue, especially in the work place, is one of toilets. Assigning them to the disabled toilets is a massive no no. I don't personally have any issues with m2f using the men's, but I'm pretty chilled. More often than not though the m2f wants to use the women's, as that is what they identify with. Fair play. But quite often a lot of the women feel uncomfortable with that... Which is perfectly understandable imo. But I've seen it twice now that the women have been described as 'unreasonable' by said individuals (and that's putting it extremely nicely).
Despite being pretty relaxed I'm on the side of the women.
I won't even go into the individual that went from man to woman, unannounced, lived as a women for two years then before surgery decided to change back. I felt sorry for his wife and kids..