Transmisogyny is the subset of transphobia that marginalizes, debases, and invalidates trans women.
Transmisogyny is the most common, most caustic, and disproportionally largest form of transphobia.
Transmisogyny specifically focuses transphobia against trans women with deadly accuracy.
This is deadly serious: Casually transmisogynistic utterances such as Whedon’s normalize transmisogynistic behavior which runs from ridicule–which is linguistic, social, and psychological violence–through harassment to actual physical violence against all trans women. Moreover, intersected with racism, transmisogyny kills trans women of color disproportionally.
This is why it is important to fight transmisogyny at all levels, even at those like Whedon’s lowly tweet, toward the goal of ending transmisogynistic violence. If I’m being harsh on Joss Whedon, it is because he has been feted time and time again for being an amazing feminist, writing strong female characters, etc. So much so, in fact, that I gaslighted myself and puzzled over whether this tweet was from the real Joss Whedon. Women are not defined by their genitals. Trans women are women, regardless of their genitals.
Some other examples of what transmisogyny looks like.
Update 2014-01-27 18:53: Irae Nicole at io9 asks the same thing I did: Really Joss Whedon? Really
Update 2014-01-27 20:08: Aoife writes a more thoughtful piece on her blog.