Well, that sounds preconceived to me. I think you're also misunderstanding me. When I said that, I'm talking about a woman who just happened to be walking by on the street. You're not expecting to have sex with her or anything; the question is simply, "Would you find her attractive?" and it seems like the answer is, "For the time being, yes."
But since you've already jumped in bed with the hypothetical person, I have another question. When you say that arousal for you requires the idea of a natural set of genitals, do you mean that if a trans woman's genitalia was indistinguishable to your eye, and you were unaware of its, er, provenance, that you would find her attractive? And that you would lose your attraction to her if you found that her genitalia was not what she was born with?
This still sounds like textbook prejudice to me. I think it's somewhat akin to thinking that gay people kissing is gross. Your grossed out response might seem completely natural to you, but it is still a learned response. Or for a perhaps less fraught example, it is akin to a fear of some creepy-crawly animal, one that you don't have as an infant or a toddler but acquire later as you grow up. If you could look at a woman without knowing whether she is cis or trans and find her attractive, and then only lose your attraction upon being told that she was trans (as opposed to you being able to tell because she didn't meet your standards of verisimilitude), I'd call that prejudicial.
I should say that as a gay man the question is somewhat different for me. Presumably the ability to reconstruct a penis has improved since last I saw images five or so years ago, but I'd have to confess that I do understand the idea that genitalia not looking "right" would be a misgiving in a long-term relationship, because what I saw wasn't convincing. I actually found images of trans men who hadn't had that surgery done to be more attractive, oddly enough. In any case, it's much easier to go your direction than mine, so to speak. And I know for certain that if trans men were as blessed as trans women can be in this particular respect, it wouldn't be a question for me.
Yes, if I couldn't tell the difference in genitals, I'd be attracted to that person, though I find such a scenario tough to imagine. Once having learned of their provenance, it's hard to say how I'd react at that point, but I'd probably be unhappy. You're aware that 'ideas' play a part in sexual attraction, right? As I said, I'd find it tough to deal with the knowledge that the pelvis I'm fucking is biologically male, that there's no uterus, that this isn't a natural situation. As I said earlier, I would not date a cis woman with fake breasts because the unnatural nature of it is far too much of a turn-off to me, and I am 100% strict / serious about that so there's a consistency there.
So it seems to me that what you are doing here is denying my sexuality and how I experience it; there's no other way to describe it. I'm telling you what turns me on and off, and you're telling me that it's learned or prejudicial with no evidence that that's the case. What if I told you that your homosexuality is prejudicial to women? It's absurd, no? Is not dating women taller than me prejudicial to tall women? Look, if I could find the idea of having sex with a trans women appealing, I would love to. But I don't. If I could find other men attractive, I'd be out there trying to bang whoever'd have me. (And probably failing miserably.) But I can't, so I don't.
It's completely different to thinking gay people is gross because what we think is 'gross' can change with education, and certainly has with me on many issues. What gives us a boner tends not to change, and certainly never has with me.