Not even because it's a lie.
I'm OK with kind white lies like "no your bum looks fine in that dress" that are only lies about the person's reality. If TWAW was that sort of a lie I'd have no issue being kind.
The issue with TWAW is that it is a lie not just about trans women but about every single woman (female person)'s reality.
It is the lie that the thing that makes men and women different is how they think not the bodies they have.
It is the lie that our minds just happen to come out different and it's nothing to do with how society treats us due to our sex.
It is the lie that our body sex has no impact on our lives but whether we think as a man or a woman does.
It is abhorrent than anyone would believe a man's need to hear he is a woman is more worthy of respect than a woman's need for her own reality.