This has been a lingering thought for quite some time, and it is now being presented here.

(Firstly, as a person with gender dysphoria, and who also supports transitioning as a helpful means for such people. This is in no way bashing anyone.)

Atheists put reason and facts, over belief and faith. That it is better to live intelligently, then to live in a lie. There has been a notion that men can biologically transition fully to a women and that women can biologically, fully, transition to men. Yet, with the bodily altering medical procedures available at this era, we are not capable of producing such results. For example, transwomen do not have a period or the chance to not have a period, and transmen don't produce sperm. That is not a full transition, and hence not becoming a full woman or man. However, very similar. Now, why would Atheists support that notion of such full transition, when it is currently improbable. Why support any lies or fictions.

Yet, how could someone resit the lies of religion, only to support a different belief? What reasons are there to endorse a different lie, one rooted in fiction?

Still standing with this, facts and reasons should govern the world, not belief.
Gender dysphoria is a real condition, transitioning helps such people, that is true. They are quite similar to the sex they transition to. What is not true, is this full, biological, body transformation.