I highlighted some of the problem phrases in that. I think it is perfectly reasonable to ban guns. You do not. I think, being a reasonable man, that this harms no one and helps many - you, being a reasonable man, think there are harmful consequences. Of course, each of us thinks the other is being unreasonable.
While the phrasing in the quote is nice, it boils down to what we can come to agreement about - in other words, as I said before, a social contract.
But, to show how deep the rabbit hole goes, and for the sake of argument, suppose we say the quote describes a situation that is actually achievable. On what basis, other than my own sense of right and wrong, should I accept it? This too would require an agreement to abide by it from all parties concerned.
So, there are two serious points of failure. The first is overlooking the fictional basis for the rule. The second is that applying the rule requires us to rely on something other than the rule.
You really should read the essay, which you obviously haven't. The essay argues that natural laws, and the natural rights which flow from them, are rooted in our biology. A product of evolution. Natural law exists because of the kind of animal a human is, which is a social animal, but not a socialist animal like a bee or an ant.
In the context of the essay, "reasonable" means normal, as in not psychopathic. The vast majority of men are "reasonable" to the extent that they can live together and cooperate and build things as humans have for tens of thousands of years. Yes, we have been taught in the schools that humans have a violent history, and I suppose they do relative to contemporary, developed societies. But that violent history has been greatly exaggerated, particularly by thinkers like Hobbes who believe that government is needed to control what is a naturally savage and irrational human. Humans are not naturally savage, although they will become so under extreme stress, which is also rooted in our biology.
In any case, by the definition used in the essay (and particularly in the part I excerpted), you are a reasonable human, as am I. The fact that you push peacefully for laws which I will oppose peacefully does not change that fact. I do not consider you a sufficiently direct and concrete threat to my safety that I would act in such a way that you might consider the same about me, and vice versa.
The right to self defense is rooted in a biology we both share, although we may not understand it or even think about it. Despite your opposition to guns, and let's assume for the moment that you want to see all guns banned and that they have been, I think if you saw a man on a street attack another man with a rock or a knife and the defending man then pull out a gun and shoot the attacking man dead, you would not find the shooter to be a threat. If that man turned to you and said "he was trying to kill me," I think you would run up to try to help and would not fear for your life from the man with the gun. And if later the police arrested that man and charged him with murder, I think you would voluntarily testify in that man's defense. That is because I believe you are a reasonable human.
Now, does this natural right to self-defense become a right to own a gun? Of course not. The right to own a gun is something that the societies we live in can either allow or proscribe, and they will still function ok as societies. Reasonable people won't immediately think the government which passed a gun ban (according to the rules, I should add) is illegitimate, as they would if the government passed a law banning private ownership of property.
However, the right to own a gun can still be argued (persuasively in my opinion) from the standpoint of the right to self-defense. A gun is a great equalizer, and it is the only weapon I know of which can deter a 250lb man from attacking a 100lb woman. As the old Colt Manufacturing slogan went "God made man, but Samuel Colt made them equal."
The essayist goes a little further and believes that private ownership of guns is necessary to prevent government tyranny. I agree with him. I am not saying that tyranny is inevitable if the government has an absolute monopoly on guns, but it becomes a greater risk, and that in my estimate, the price we are currently paying to minimize that risk, in terms of innocent lives lost per year because of our 2nd Amendment, is worth it. That's not even counting the ancillary benefits of private gun ownership, which is a general reduction of crime, both violent and otherwise (although obviously not murders with firearms).