'Being explicitly excluded, to trans people, is just as insulting and denigrating, and will react to that accordingly by decrying disclaimers about "not dating trans people" as transphobic.'
This is about sexual attraction. Sexuality.
Homosexuality has been in some countries and still is. Illegal. Socially so unacceptable that there are large numbers of beatings murders. Gang rape and murder of out lesbians. Some countries have death penalty.
Beyond that in countries which enforce narrow gender roles past the risk of being deemed homosexual, the freedom of especially female children and adults is highly policed. Arranged marriage, child marriage.
I may be making an assumption here but I'd imagine that in general in a forced marriage the girl/ woman. Well in theory her husband has marital rights and in practice i have no doubt that a gay man in this situation will have a shit life but pressure for intercourse a different dynamic. But a lesbian will be in no position to refuse. And frankly in that situation sexuality is by the bye because rape. Women and girls have way less freedom to get away by themselves/ meet other people etc etc. They are property.
For trans people in those countries where I doubt there are explicit laws, presenting or in any way being seen to be too feminine or masculine will be interpreted as homosexuality and same oppression applied.
On that note marital rape was only criminalised in England and Wales in the early 90s.
Your point is that in more liberal countries, asserting your sexuality and at times bluntly saying no is transphobic.
Given the history for lesbians, gay men and bisexual people who in any way gave away the same sex attraction part of their sexuality. In the UK. And still desperate all over the world.
And given that it was legal for a man to rape his wife until the early 90s- so recent. And that all over the world millions of women and girls have no choice about who or when they have sex (rape).
Given that where women have the legal right to say no and rape is illegal, and gay men and lesbians and bisexual people are protected in law, it has been FOUGHT FOR in living memory.
Given all that.
What gives you the idea that it's reasonable to compare exclusive same or opposite sex attraction as like RACISM...???