I think you should be more careful with this kind of argument. You start by invoking "transgender identity" (your own words) to distinguish it from sexual orientation. So far so good. But you end by stating that "there is no reason to make a distinction between trans and ciswomen that isn't based on transphobia".
Because there isn't. Sexual orientation has nothing to do with gender identity in this context. Humans do not possess a hidden transceiver that can detect if somebody is trans, intersex, cis, etc. Sure, we have an idea of what is masculine and feminine constructions (due to social constructs), which might cause some trans people, particularly those pre-HRT, to have red flags. But those that have been hormones for a significant period of time, that have features that match the gender they're transititioning to? The only way you'd be able to tell they're trans is being told.
I don't think anyone defines her own identity as "ciswoman", that is not a thing, women thing of their their gender identity merely as "women". Trans(woman) is a different category you are using, and one that you go as far as defining as an identity, something that sets you apart as a member of a restricted group.
Ok, if you stick to this argument then you can indeed claim that not wanting to date a "transwoman" is "transphobia". But you also must agree that a "transwoman" is different from a "woman" in that it has an identity (transgender identity) that a woman does not have. Is this really what you wish? Doesn't this hamper the demand of, as a transwoman, being treated just a woman would? I mean, what one's partner identifies as is rather important in a choice of partners. We do have many "identities", whether or not a particular one matters, and how it does, is up to the people involved. But it seems reasonable that it can make a difference for some people.
OK I think I need to clarify something because something is not being properly expressed. I'm basically arguing the cis vs trans dichotomy is a social construct. I'll even explicitly say it: the cis vs trans dichotomy is a social construct.
However, there is a common misconception from people that when people say that something is a social construct, that it means that something isn't real. Gender divides exist in society. Racial divides exist in society. It takes a really willfully ignorant person to argue that people aren't treated differently because of their gender or race or whatever else that has been called social constructs. The term social construct instead means that something is
artificial. As in, that thing being called a social construct has only bearing whatsoever because we as humans assign value to it beyond any objective biological purpose. If we as people stopped assigning any value to that particular concept, it would cease to exist
at all.
So yes, in the context of my argument, I am saying that, currently, presently, in society, there is a meaningful distinction between transwomen and ciswomen
solely due to the way society treats the two groups differently (as evidenced by this thread even existing). However, no meaningful physical distinction differentiates an infertile ciswoman with a transwoman. Therefore, I reject the idea that trans and cis women
should be treated differently due to their gender identity, and that the social construct shouldn't exist. However, I am not a society (yet

), so I unfortunately do not have the power to simply erase social constructs unilaterally. The best I can possibly do is argue against oppression based on imaginary concepts by trying to show the underlying issues of a particular viewpoint. This, of course, requires me to engage in a worldview where the social construct exists, in order to show why it shouldn't exist. Thusly, I have to use the distinctions to argue the distinction doesn't exist.
Does that help? Do I need to clarify anything else?
Some thinks are just facts of life, and one is better of accepting them and moving on taking them into consideration, that fighting reality. Perhaps there are "transphobic" people in the sense that they wouldn't engage with one. So what?
There is a lot of toxic concepts melded into this one section that I don't know where to even begin here.
1. This is definitely off-topic, but given the current presently accepted models of universal phenomenon (general relativity and quantum mechanics), I find it hard to believe that there is such a thing as a "fact of life". Everything is retaliative to our ability to observe and comprehend, and in the field of physics we're already seeing those limits break down when we deal with scales far above or below what our biological sensors are built for. For society, which is ultimately created by humanity itself, this is even more true. There is nothing that forces a person to act a certain way beyond the person itself (this is true whether or not you accept free will. If you don't, then just add a subclause saying that its the neurochemical reactions internally within the person which causes a person to react a certain way). Therefore, there is nothing that forces people to act a certain way beyond the people within the group. Therefore, if the people who comprise society changes, the rules of society will necessarily change with it.
2. Regardless of point one, "accepting" oppression is being complicit to it. I refuse to act as an actor of my own oppression, so no, I will not accept the fact that its ok to discriminate against me simply because I am trans. I believe society can be better than it is now, and I'll be damned if I don't fight for it, in any way I possibly can. I may not be an effective actor of change at the present, and engaging in discussions may not be an ultimately effective tool to outreach people, but I know I can do this, so I do.
3. So what? The so what is that I'm treated like a second-class citizen. Don't be patronizing.
An interesting question that arises from this is: if ever sexual reassignment surgery could 100% change someone's body (let's ignore DNA, it's not really relevant), would it continue to make sense to talk of "transgender identity"?
No, see above. Can't wait for that day to come.