I mean it’s pretty straightforward - due to trauma over her past experiences with sexual assault at the hands of cis men, she now sees trans women as a facade used by predatory cis men to sexually assault cis women in bathrooms and locker rooms and other segregated spaces.
To her, trans women are really cis men pretending to be women, to make it easier to rape them. There’s kind of no nice way of saying it.
It’s textbook transphobia / queer bashing. Fear of sexual assault at the hands of queer people is probably one of the most basic reasons to justify this particular brand of bigotry. “I don’t hate queers, I’m just concerned for the safety of -“ take your pick - women, children, sometimes even men. For JK it’s women.
Ah but what if I don’t subscribe to your belief system?
‘Man’ js the word tradition taught you to describe an adult person who is male - but what does that mean, really? Is it a person with a penis? If a man’s penis is removed, what is he then? Prefer his genitals at birth, maybe? What if he’s born without a penis? Intersex people exist. Chromosomes perhaps? There are all sorts of extant combinations beyond XX and XY.
The real question you should be asking is, why does it matter? how does this belief in men and women serve you? It seems to me like your insistence in following this tradition is actually hurting you, not helping you, because it’s narrowing your understanding of your fellow humans, to the point where you can confidently say things like “it’s because they’re male obviously” as if that doesn’t make you look incredibly foolish by modern standards.
At the very least, you may want to consider keeping your belief in gender mythology to yourself.
The idea that men can become women by self-decree is a niche ideological view. It's quite a stretch to call this "modern standards" when hardly anyone really believes this.
Your hypothetical male who lost his penis or was born with an underdeveloped one is clearly still male despite this. There is a misogynistic tradition going all the way back to Aristotle that women are merely deformed men, but in this day and age I hope we can move on from such ignorance.
The alternatives - that "man" and "woman" are identities that anyone of either sex can claim, or that "man" and "woman" are defined by a narrow set of cultural stereotypes - are very niche definitions that should be disregarded as, respectively, absurd and sexist.
To her, trans women are really cis men pretending to be women, to make it easier to rape them. There’s kind of no nice way of saying it.
It’s textbook transphobia / queer bashing. Fear of sexual assault at the hands of queer people is probably one of the most basic reasons to justify this particular brand of bigotry. “I don’t hate queers, I’m just concerned for the safety of -“ take your pick - women, children, sometimes even men. For JK it’s women.