They fail to understand that secondary sex characteristics in non-transgender human beings are indicators of our primary sex characteristics. Up until very recently, significant facial hair or lack thereof was a clear sign that that person had male sex organs or female sex organs respectively. Sure, it might be a bit of a mindfudge to see someone like Buck Angel, who is part of the 0.01% of transgender-identified people who pass with clothes on, naked, but it's ultimately irrelevant in the grand scheme of human sexual attraction.

It's also interesting and awfully convenient the way truscum frame sexual attraction. To them, it's simply a matter of how well one can pass as the opposite sex (again, as long as the clothes are kept on), completely ignoring the fact that androgynous people exist. Remember the tweet about an androgynous-looking gay guy kissing an androgynous-looking lesbian by accident (https://i.imgur.com/bQD2o0D.png)? Alphabet soup activists went ballistic, demanding that he "explore" his orientation and be more "open minded" and he just laughed at them and told them the facts: they were just two homosexual people caught in a hilarious and embarrassing situation. Stuff like this angers them because it shows that just because you superficially resemble a person of the opposite sex doesn't mean you are actually a member of that sex. Just because you spent your life savings on a suit, expensive shoes and a Rolex doesn't mean you are a millionaire.