(Same thing for being squicked out by women with dicks.)
How exactly do you define a woman? That's what I don't get.
My definition is [someone with functioning ovaries]
Why don't you try thinking about what it would feel like to grow up knowing in your heart that you are a woman, trapped in a male body, then discovering there's other people like you, learning that there's a way to actually become what you've always felt you are. Now imagine that after starting to transition you meet someone who tells you that you aren't a woman and will never be a woman because your body doesn't produce eggs and never has. How would that make you feel?
"Knowing in your heart" isn't a real kind of knowing. It falls into the same category as blind faith, divination by the casting of lots, automatic writing, answers in Genesis, and DMT fueled "revelations".
EDIT:
How you feel about something and what you know in your heart are irrelevant to and utterly impotent to alter the facts of a situation; any situation. A simple tour of a madhouse should be sufficient to make that clear.
I think you're ignoring the point a bit, if someone is being an asshole to someone else for their lifestyle choices, that makes you an asshole, same with insulting vegetarians, or vegans, or people who abstain from sex until they're married, and that whole arm metaphor thing doesn't fit, either, I'd say it might be s'more like... I dunno, having feathers instead of hair, it would just seem wrong to some people who had that.
And I guess someone feeling wrong in their current gender isn't unique, precisely, I'd say it's the same kind of gut feeling that makes people want to change their hair color, or get jaw reassignment surgery, just... A little more complex.