Sex and Gender are completely different things.
Transgender people don’t necessarily undergo a sex change - mainly because sex isn’t changeable (ie XY/XX chromosomes). Transgender simply means you don’t identify with the gender (a made up thing, gender doesn’t exist in nature) you were assigned with at birth.
And for many people -- especially those in marginalized communities -- they get the latter reaction so often that they understandably get a little preemptively defensive about being stepped on.
Sure, certain connotations can change rapidly, but that's why I try to listen to those that are actually affected by these words one way or another. It's pretty much literally the least I can do.
Usually, when a trans* person or another minority make a correction like that, they're not looking for any sort of big thing. They know mistakes happen, and that's fine. In fact, they generally do NOT want you to make a big deal apologizing or anything, because that's ALSO really uncomfortable. They just want you to try and remember it for next time. That's all.
they SYMBOLIZED his body and blood that he was about to sacrifice.
This is all very new, counterintuitive, and opposite to what even the most PC people said 15 years ago, so, some are understandably confused.