Because their hatred of women is what informs their trans and homo -phobia as well. If you disdain women, then in their minds, a transgender woman becomes a man who chooses to no longer be a man, instead "he has rejected manhood and wants to become" the very thing they most detest - a woman! They hate women so much that anything that is associated with women from special days, to clothing, to foods, and activities women enjoy, is also worthy of disrespect! Their hatred of women informs every aspect of their online persona. They craft entire identities around it.

Since they think cis-women exist for only one reason, "sexual availability", a trans woman, in their minds, is a gay man who pretends to be a woman to trick straight men (themselves) into having sex with them. They're deeply disturbed at not being able to determine if a woman is transgender because they believe sexual attraction or activity with a transgender woman makes them gay, and they hate gay men because they hate women!

It's a tangle of hatred really!

https://www.opendemocracy.net/en/5050/misogyny-and-homophobia-patriarchy-gender-policing-and-male-gaze/