This quote is from an essay on the Good Men Project website, so it's got the caveat attached that it's by a cisman who is trying hard to be seen as an ally.
I thought this part was worth repeating, though:
When encountering a transgender woman, many cisgender men don’t see a person. They see a mirror. If they are full of questions about their sexuality, when they look into the mirror all they see are their questions. “Why am I attracted to this person? What does it mean? Am I gay? If I’m gay, am I still a man?” These questions fog up the mirror so that a guy can’t see himself clearly. It’s dangerous business bringing questions to a mirror unless you’re willing to open your eyes.
(alias-pseudonym points out that "maybe it would be more accurate to say that cishet men normally take a huge number of things about their identity for granted and therefore feel overwhelmed when their assumptions are challenged in any way". But if they aren't het or cis and haven't realized it yet, that also means we remind them of the alarming parts of themselves that they haven't examined yet.)
You can see, in this same essay, where he looks for us to reflect what he wants to see in us, using poetic language to compare trans women to myths about a nonbinary deity. I'm not sure he realizes that trans women and men tend to see ourselves as the gender we're transitioning to, full stop, and that being told otherwise is extremely offensive. Or that trans folk who actually are nonbinary and/or genderqueer often face prejudice and incredulity, even within transgender communities.
Either way, comparing us to mirrors feels appropriate, especially since you don't choose to become a mirror. You're just stuck in one, whether you like it or not.