In my previous comment here, I mentioned there’s a longer answer to this, so here it is (in addition to what I said there).
Let’s address the following:
They think people are attracted to each other based on gender identity, and if you're attracted to someone with the same gender identity, you're gay. If you're only attracted to those of the same sex, then that's a preference and not an innate sexual orientation according to TRAs.
The truth is that it’s complicated. Both physical sexual characteristics and gender identity matter. As does gender presentation.
I’ll explain… and I hope no one will find this uncomfortable, since I am going to be pretty direct and dry with my thoughts on how physical attraction works, especially for myself. If anyone finds it problematic, please do tell me.
Firstly, you have to keep in mind that there are people — granted, a minority — who are demisexual. For them, the body doesn’t matter (much) — only the person within it.
For the rest of us, the body of the other person does matter, and often needs to be appealing in order for us to be attracted to them (at least initially). But while sexual characteristics are important there, they’re not the end-all & be-all even for us. Indeed, considering how transgender people fit here is quite helpful when thinking this through.
For instance, let’s look at me as an example. My attraction is towards women (in the general sense), and femininity appeals to me. Conversely, men don’t interest me sexually or romantically.
And what about transgender folks? Well, it depends.
When it comes to trans women, my criteria there are the same as for cis women. I don’t care what ‘equipment’ they were born with; what matters is what they’re like now, and perhaps the direction they’re going in.
Unfortunately, this means that pre-transition trans women are probably not gals I’d be in an intimate relationship with. Not because I don’t think they’re women, but simply because they don’t appear as such when it comes to body, voice, perhaps the clothes they wear etc. At that particular point, they’re likely outside the limits of what I find attractive, limits which stem from my sexual orientation — something I can’t change.
However, as I said, physical sex characteristics can be changed, and trans women often do end up changing them, sometimes by quite a lot. What this means is that while they may not have held any sexual or romantic appeal to me before, once they’ve gone some distance in their transition that might easily change. And I don’t mean that they have to “pass” as their gender 100%; rather, as long as they read as “sufficiently physically female” to my perception — as subjective as that is — I might find them attractive, even if they’re somewhat “visibly trans”. Again, attractive in the same way as it goes with cis women — as I don’t feel any sort of 'ickiness' at the mere thought of someone being trans and/or non-binary.
Now, an interesting thing to consider is: what about trans men? After all, according to the OP, I should be attracted to them, right?
Well, pre-transition, that might be the case. But see, that’s just the initial, surface-level feeling. I’m the sort of person who’d want a deeper connection and relationship with a partner, and while I might perhaps find a pre-transition trans man’s body attractive, a relationship with him wouldn’t be possible. Why? Yep, it’s because gender!
Think about it. This would be a person who feels like a man and tries to act male, so to speak. Someone who likely wishes to medically transition and change their body to be as masculine as possible. That means any sexual relationship with them would basically be “doomed”, because we would soon end up being incompatible due to my sexual orientation. Hell, the whole thing probably wouldn’t even get started in the first place, as the guy would likely be presenting masculine already, and that’s just not something I find appealing.
So, to conclude, it’s ultimately a partner’s gender identity that matters most here — because while I might not find a trans woman attractive initially, that can change when she transitions. Conversely, while in theory I might have an attraction towards a pre-transition trans man, that will quickly fizzle out when I find out what his real identity is, especially if he transitions.
Well, that’s my thoughts on humans for you.
As for what was mentioned regarding animals, well… I’d say they don’t really matter for our considerations here. Animals are so varied and so different from us that we really can’t be compared much with them. After all, for one, there are species that are functioning hermaphrodites, and species that reproduce completely asexually. Not to mention that the sheer variety of sexual behavior, physical traits, reproductive methods and sex roles among animals is staggering. Finally, we are very different from them because of our full sentience and the vast range of social and psychological consequences that stem from it.
I hope all of this makes sense?