'Transwoman' is transphobic because it's a noun, in the same way that 'woman' is a noun. Thus it draws a separation between women and transwomen, automatically implying that transwomen are something other than women. 'Trans woman' is a noun modified by an adjective, much like 'tall woman', 'fat woman' or 'black woman'.
Of course, the only women who are capable of being trans women are the ones who started off male. I use 'transwoman' in left-leaning spaces, because I'm not looking to start chimpouts (if I discuss trans stuff elsewhere I engage in zero trolling - I'm looking to get people to consider reality from a slightly different perspective, not rile them up by yelling 'tranny tranny tranny'). It's not being directly rude, most trans people/allies won't call you out or push you on it, but they are aware that while I'm happy to discuss the issue civilliy I'm not going to be pushed into the doublethink that 'trans woman' implicitly requires you to express.
Thread tax: literal incel attitude from a HSTS
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Why do men prefer women who treat them badly rather than dating a
nice guygirliepop?