I don't really know if it's transphobic tbh? If it is because the things that you said (not seeimg them fully as men and therefore not seeimg them as potential partners) then yes, absolutely, but it could also be that these particular men are just not your type?
It can be really hard to leave the transphobic point of view we usually grow up with and hold trans people to the same level and expectations as cis people (I am agender and I struggle with this quite a lot). Smth I do is imagine scenarios of the thing that I have trouble processing and I imagine a cis person doing it, and then I imagine a trans person doing it and I tell my brain that there's no difference.
Obviously that's an oversimplification: there IS a difference between trans and cis people, as they would have different experiences and would be prone to doing different things, but that's how it is with everyone, so really it's not that big of a deal if it happens to be a trans person. Of course, in a sexual situation the difference would take a major rol, I think, so I cannot really help you there. But maybe if little by little you start holding trans men to the same standards as cis men (not only in romantic or sexual scenarios, but in everyday life), a change could be made.
Also, it's absolutely okay if you never get a crush on a trans man, you can't really choose the ways of the heart, after all (I've learnt that the hard way lol). In any way, it is really cool of you to ask this and actively try to make yourself a better person, I wish you luck in your dilemma! 💛