I'll bite the hook. I don't think it has much to do for most (though certainly not all) with a resistance to the future, but rather to a sort of "uncanny valley" scenario. Certainly if a trans woman were post-operation and in general respects was visibly and audibly indistinguishable from a cis woman, you'd see a lot less hetero-identifying men having a fuss over what they've just seen or heard. Indeed, were they not even informed that the woman was trans vs cis, they ought to have little basis to contemplate it, if that's the case. But the issue lies in that we've yet to achieve a process of transitioning which yields a consistently indistinguishable outcome, and for different individuals there are different thresholds that will trigger an uncanny valley discomfort reaction (in the same way as happens with the original use of the term, that being artificial constructs intended to simulate a human).
If you're not familiar with the term, the uncanny valley refers to the period in between "it is definitely not the thing that it's being claimed to be" versus "I have no logical reason to question that it is what is claimed". For something like a robot, this could be things like jerkiness in movement - a clue to the mind that the thing being seen isn't a person, even if it's being presented as a person. A great example is the scene in Terminator in which the titular character has to self-excise a damaged eye. Although the fake Arnold Schwarzenegger head is very well-constructed and looks quite good for a model prop, it doesn't quite cross the threshold into being unquestionable, yet nor is it so questionable as to be obviously not what it's meant to be, either, for some people, and for those people it will cause them to feel uncomfortable.
So, circling back to transitioning: while it's definitely fair that there are some folks who are simply bigoted individuals, there are also plenty (like me) who are supporters of trans people but who nevertheless tend toward not finding themselves attracted to trans individuals of the gender identity they normally like, on the basis of the individual in question happening to fall into an uncanny valley situation for them. This is especially easy to have occur once clothing comes off, since even post-operation, it's a very rare... for lack of a better way to put it, "crafted vagina"... which is a strong likeness to a natural one. That, too, can be an easy uncanny valley situation trigger for many. I wish it weren't the case, and that we'd perfected the means to make even the beefiest stud of a man into a dainty and indistinguishable damsel (and vice versa), but the science and surgery isn't quite there yet. Someday it probably will be. But in the meantime, we have to accept that even well-meaning allies may feel discomfort due to no fault of their own - nobody chooses to be affected by the uncanny valley effect. It's just a part of our robust set of instincts that biologically give us caution against dangers such as a camouflaged predator.
