I guess that is a hard hypothetical to answer because absolutely the most disturbing thing would be the lack of honesty and the feeling that I was tricked. An also I think it would be hard to prove that at this point a trans women would be indistinguishable from a natural one. It may feel off during the act, and after they disclose it may then makes sense why it didn't feel right, and I would feel violated. Put this way, if a girl blind folded me and said she gave me a blowjob, then switched out for a man to do it, I would absolutely be disturbed even if it felt indistinguishable.

If the answer is something along the lines of "it just does" or "just knowing she used to be a man," it may be worth examining whether or not one of your reasons (consciously or not) is that you still think of her as a man.

My reason is absolutely that I still think of her as a man because she biologically still is, and that is a more important factor than outward appearance. No one has any right to make that decision for me, not disclosing it is straight up dishonesty. Again, this isn't bigotry unless you think people are not entitled to their own sexual preferences.