If you meet a nice "lady" and you two start getting physical one day and it turns out this chick has a dick how would you react? I would probably punch her/him in the face. While i'm not condoning murder but one punch which could count as assault or battery would be okay to use this defense.
For the most part, you can usually tell a trans from the real thing. I have not been fooled in the past but can imagine my horror if I found out they were a guy after becoming somewhat intimate with them. Not being prone to violence, I think I would at least push them away and use very disrespectful language toward them but I think this minor physical and verbal assault would be forgivable. For people less restrained, I could see them become a bit more physically abusive but as long as they don't cause permanent damage, that too may be acceptable. After all, the person mislead them to cause a physical response so they should not complain when that response is not to their liking.
It is wrong to blame your sexuality or attractions when you commit a crime. If you murder someone because they tricked you into making you believe that he is a woman but he is actually a man then sure go ahead and sue him and maybe even punch him, but if you murder that guy because he tricked you, you should go to jail.
The gay or trans panic is basically confessing to a hate crime and using it as an excuse. To say "I found out they were gay/trans so I automatically attacked them" clearly shows that you are dangerous because you will attack people based on what comes down to a political disagreement. That's not a defense, it's a further indictment of yourself.
Why would I be able to use something like sexual; orientation to defend myself when being charged with a crime. If i committed a crime and then was pardoned from all consequences because of my gender/sexual orientation I feel that it would ruin our judicial system permanently because now you create loopholes for people who aren't in these "minority" genders to get away with whatever they want.
It is not fair because they still did the crime and should not be offended to the point when they do a crime. They should not be treated diffrently because they are gay or trans. They should be given the same punish meet. Also they don't like to be treated diffrently so give them and equle punishment.
People are using the excuse to get out of a crime, even if they are gay/trans they shouldn't be able to have an advantage over people who are not gay/trans. Right now people are trying to make gay/trans people be equally treated just as non gay/trans people but they are just basically pulling out the gay/trans get out of jail free card which makes it harder for people to trust gay/trans people if they are going to take advantage of there condition.
I can see why using it is popular but it is your fault for the crime (unless you are framed) but saying, "I did (action) to (person) because they were gay/trans". Is just down right wrong. Blaming someone for their gender or sexual beliefs is just bad. Bob bob bob, (sorry i needed more words to post this thing).
Okay, so say that you and your gf got a tad bit too close, and it turns out that they have a dong, and you shout 'oh shit' and reflexively slap her/him/whatever. Saying 'I panicked because I wasn't expecting a dick' doesn't really excuse the fact that you still pretty much slapped him/her/thisisstupid, and while they probably should've told you first, it still makes you a bit of an asshole.
Now, I get that it may be justified in the odd extreme circumstance (not condoning this kind of thing, but there's an exception to every rule) but in the vast majority of cases, it can easily be resolved by a simple 'oh. I wasn't expecting that.' or something like that, and then you both have a good laugh over it in the pub an hour later.
Something like that. TL:DR there's no need to slap them.