There are 19 states that impose some "duty to retreat".
It is not reasonable to think that deadly force, or any force for that matter, is a justified response when "any" physical act is visited upon you.
However, a duty to retreat is very complicated and requires a jury to agree with your point of view; whether you are the prosecutor or the defendant.
I teach defensive firearm classes and concealed handgun permit classes. One of the things I teach is that if you're paying attention you should never have to draw your weapon and, if you do, it is unlikely you will have to fire a shot. However, students always raise a ton of "what if" questions.
I always answer the "what if" questions by explaining that if you're able to stand there and objectively go through a check list justifying why you're in fear of losing your life or of grievous bodily harm then you are probably not justified in using deadly force. When faced with impending death or serious bodily injury you will not be analyzing legal options; you will be trying to survive. When trying to survive, the firearm becomes one of many tools available to you to aid in your survival. Another tool includes tactically retreating.
I apologize when someone bumps into me even when it's their fault. I back away from aggressive drivers to avoid road rage incidents. I tend to be quite deferential to jerks and their rude behaviors. I want to avoid trouble and I go out of my way to make sure I do. I understand that someone might be acting like a jerk because they've had a rough morning or are just having a bad life in general. They aren't my problem and I will do whatever I can to keep them from becoming my problem.
On the other hand, if someone punches me it will be difficult for me to believe it is anything other than the start of an ongoing attack and will do whatever I have to do to survive. If increasing distance from my attacker is possible then I will do so because it is the safest, most efficient way to stop the immediate threat and to ready myself to respond with greater force if necessary.
In force-on-force simulations it is not unusual to see someone "run away" from the danger presented. The analysis and de-brief after the exercise centers on whether the person being attacked ran away in the safest and most effective way possible. The de-brief doesn't include admonition on why the student should have used their gun to stop the attack. Running away safely is a valid tool to survival.
I get nervous and become uncomfortable when someone, even joking, threatens violence. I winced when I read the parent post that started this discussion where the poster said, "doesn't mean I can't punch someone in the face." That attitude of justifiable violence for being offended is what scares me and it's fairly prevalent in these comments. Then again, I suspect that most of the people advocating a punch to the face are, as you call them, "internet tough guys" and, I hope, wouldn't actually commit the act of violence they are so quick to advocate.