I think there's another aspect that's often overlooked. Surgery.
Your average left aligned normie hears on the internet that trans women can have vaginas that are utterly indistinguishable from the real thing, to the point not even a gynecologist would know without getting out the speculum. Trans women were born in a male body, sure, but you literally can't tell at all as they look exactly the same as cis women. So if you don't want to have sex with a trans woman, then you must be a raging bigot because you'd only know if they told you.
Now we know that isn't true, because of the wide variety of troons on here with mangled amholes who don't pass. But that's what (lefty) people believe. Meanwhile there's not a widespread belief that trans men have fully functional dicks that look just like the real thing - some may claim it, but for the most part people either think trans men can't get surgery or they have their own stuff going on.
And that was the thin end of the wedge for stuff like "girldick", but not the other way around.