That's 100% not my username, haha.
There isn't an appropriate response. I believe that the trans person should divulge that they're pre-op trans before getting naked, because it's courteous. It's going to be a surprise either way, but it's courteous to lesson the blow. If they don't do that, and just whip out their dick, then I suppose that incredulity/surprise is to be expected from the guy.
I'm a firm believer in trans-rights, and I support them fully, but I also don't think that people who aren't attracted to trans people should be tricked into sleeping with them. It's not okay to limit the rights of trans people, but it's also not evil to not want to have sex with them. It's not perfect that trans people don't have the same sexual liberties that non-trans people do, but I think it's unavoidable, so long as a large proportion of the population isn't attracted to them.
All of that said, violence is absolutely not justified. Sure, I can understand being angry because being angry is a natural response to being tricked, but you have to be responsible for your actions. It's fine to no longer want to have sex, it's fine to be let down by the situation, it's fine to be angry, but it's not fine to be violent.
It's also not a gray area, at all. Assault is assault unless it's defense. There's no "well, I was really pissed so it's okay lol" clause.