Those willing to use violence against a person who has not offered them violence first negate the right to have minimal force applied.

An old woman and a body builder get into an altercation at a bus stop. The woman, out of frustration, strikes the body builder with her purse. If the body builder defends himself with all of his strength, and grievously injured the woman, would you say he reacted reasonably and responsibly?

There are no fair fights. Nature demonstrates this daily.

Being creatures of insight and reason, should we not hold ourselves to the ideals of those very virtues and benefit from the dignity they give us? Nature demonstrates a lot of things. My dog, left to its own devices, would gladly roll around in rotting garbage. It's absolutely natural for a dog to do that. If you or me decided to roll around in garbage though, can't it be argued that our behavior is unreasonable and does nothing but debase ourselves?

Edit: Hyperbole aside, I think it's important to point out that the issue of force in self defense is an issue that requires sensitivity in thought. Not just because it's a delicate matter where emotions can run high, because it is, but because this is an issue that philosophers, judges, and legislators throughout the world and throughout history have struggled and continue to struggle with. People's lives are affected by this problem day in and day out, sometimes in unimaginable ways, and we need to keep that in mind. That there are no easy answers to the question and that many resolutions and conclusions seem incomplete and unsatisfying only illustrate how complex the problem is. Because of that, taking positions of extreme absolutes and blanket conclusions is not the way to go.