Of a thousand people, only a very few might want to have sex with me. For many of them, they would feel an obligation to be faithful to a partner. For many more, I would be the wrong age. Even if they would not want sex with a trans woman, or not with a post-operative trans woman, that might not be the main reason. People have varying strengths of sexual response, but I can imagine that most would not be that into me. You need to feel it to want it.
Because most people in any group would not want sex with me, I tend to feel that need not be said. I can accept it that people simply like my company without wanting more; I do not find it an insult that they do not want Heughmagandie. I don’t want sex with them, either.
This is why lesbians saying they would never have sex with a trans woman, or a pre-operative trans woman, is transphobic. It need not be said. When I meet someone I expect they will not want sex with me. If they feel the need to say so, it feels vaguely insulting, either assuming that I do not accept their non-interest, or implying I am uniquely repulsive: it’s not just that they don’t want sex, but sex with me would be impossible or unimaginable. “I would never have sex with a transwoman because I am a lesbian” is saying I am a man, or at least not a woman, and I find the statement that I am in no way a woman insulting. I am culturally a woman. That I am generally treated as a woman makes my life bearable. There is no need to say it. It is transphobic.
Possibly, a trans woman has come on to that lesbian, and not taken no for an answer. That is an unpleasant experience. I sympathise with the lesbian in those circumstances. The trans woman has no right to behave like that. But imagining that other trans women will behave in the same way because one has is transphobic. We are not all the same.
Many lesbians have had relationships with trans women. That does not make them not lesbian.
Transphobia can also make someone imagine that she will never fancy a trans woman. Generalised trans phobia might mean others never check us out, or try to get to know us.
The origin of the term was the cotton of women’s underwear, that we would never get inside. I find it an unpleasant term; I am demisexual gynaeromantic (ie not that interested physically but wanting heart-connection with a female partner) so the term seems a bit coarse to me. But it was one seminar by Planned Parenthood, with seven attendees. Here is a history of the term, and the transphobic overreaction. If you google “cotton ceiling” you find lots of TERF stuff, a lot of which is quoted on that link. It’s pretty horrible. We never meant any harm. We never did any harm.