I think there are some differences. We can measure sexual attraction. The observable effects of physical arousal are fairly well documented. The claim to be attracted to people of the same sex can be observed as true by anyone at all. The argument given for homosexuality being classed as a deviancy was that it was "unnatural", not that it wasn't observable fact.
Additionally, the cultural change required to stop stigmatizing homosexuality was minimal. It required protection against discrimination in employment and services (which should be available to anyone on pretty much any attribute or belief), it required a removal of religious perspective from the definition of marriage from a legal viewpoint.
It did not require removing a definition of sexuality, or a redefinition of attraction at all. It didn't obligate other people to change their beliefs. It didn't require other people to accept an internal feeling that was in opposition to observable reality.
In contrast, gender identity cannot be measured. And the physical indicators of sex, which are measurable, must be ignored and made unimportant for gender identity to be accepted. It relies on an internal view that is undefined and cannot be explained in any objective fashion.
Transgender people should have legal protection from discrimination in employment and services. But that's not all that is being asked for.
The definition of gender adopted by trans activists requires that the definition of "woman" have no meaning. It requires enforcement of language use. It requires that everyone alter their beliefs and their understanding so that male-bodied people who present as male be accepted as "just as much of a woman" as a female-bodied person who presents as female.
Affirmation of your internal identity requires me to erase my identity as female.
In the bad old days, nobody but the worst kinds of bigots and deniers were claiming that, for example, homosexual men were
pretending to be sexually attracted to other men.
Rather, the claim was that a) homosexual men
were genuinely sexually attracted to other men, but that b)
this (genuine) same-sex attraction was the product of a mental disorder or deviancy.
In other words, those men were mentally ill, and their mental illness was manifesting itself in making them believe they were sexually attracted to other men. In other words, these were simply "normal" (ie heterosexual) men who had unfortunately become ill*.
And of course, under that misguided "analysis" of homosexuality, your tests regarding sexual arousal would indeed show that (eg) the homosexual man
was genuinely sexually aroused by other men. The problems arise when it comes to ascribing those desires: in a modern, enlightened age, we now recognise that those genuine desires are an authentic, valid condition in themselves; in a previous, unenlightened age (which was not so very long ago, remember), they believed those genuine desires to be nothing but the product of a mental disorder.
* Much the same as the man believing he is King Henry VIII: it may be undeniable that the man does
genuinely believe himself to be that long-dead English monarch, but in that case it's also a total certainty that this belief is the product of a mental disorder.
