The narrative of these defenses is basically something like this: "And then I went to bed with her, intending to initiate a sexual relationship. As she undressed, I suddenly noticed her genitals not being what I expected. I realized it was a man trying to force me into gay sex under false pretenses, so I got scared, panicked and as a matter of self-defense against this devious attempt to sexually assault me, I took my gun and shot them. I was under stress and in shock, so I was not thinking clearly."

The idea is that you're so scared that "a man" is trying to "trick" you into "having sex with a man" by "lying about being a woman", that in that moment of panic and helplessness, you would be allowed physical, up to lethal self-defense, like in any other rape case.

The entire narrative hinges on the idea that trans people intend to mislead people into having sex with them by "pretending to be a different gender than they really are", and that they seek out "normal" people to deceive into sex, for their fetish. Which is, by the way, also the origin of the slur "trap".