There really shouldn't be any reason to not be honest about sex if it comes up and the subject can't be avoided. Hostility maybe arises from the interpretation of a person's perception of another person's motives and intent, if they feel they're being slighted or insulted, plus their own unique history that led them to being trans. Some trans people probably feel deeply insulted when their biological sex is acknowledged, others probably just feel hurt or sad, others may feel absolutely indifferent, and perhaps some even feel proud about it. It's why there are a lot of different reactions and not all trans people will react the same way when such things are mentioned.

For those who do react very negatively to having their sex acknowledged by others, I would think the reaction is coming from a place of pain, whatever the reason. Whether it should or shouldn't, it must somehow hurt to have that brought up for such a person. Maybe it's the pain of a control-freak not getting their way, maybe it's the pain of not being seen through the lens of one's gender identity, maybe it's the pain of a reminder of a traumatic life, maybe it's the pain of real or imagined fear that one lives with that their status or history will be discovered.

From observation, people who struggle to fit in after transitioning or coming out tend to react more negatively to acknowledgment of their biological sex. If they were secure within themselves, why would there be any real reaction to that fact? That's not to say that these insecure people aren't who they say they are, but they're lashing out because they feel threatened by that fact and/or it being brought up. Their biological appearance and functions betray their birth sex more readily and it's harder to blend or be perceived by others as the opposite sex (or whatever sex or gender they see themselves as), and they know it because everything in reality reaffirms their unfortunate circumstance.