Such citations are hard to come by because the equal force doctrine is embedded in common law, and only rarely codified into statute. However, it is still active in legal matters, even without being codified.
Yes. I am drawing a distinction here between the idea of the "force pyramid" or "force continuum" that sworn law-enforcement officers may need to follow somewhat closely as they
enforce the law, and the much more truncated escalation path that a private citizen would need to follow when lawfully defending him/herself.
Sometimes we err in assuming that, since cops are equipped, trained, and required to move through a variety of steps of increasing severity as they apprehend and otherwise deal with hostile suspects in criminal atercations -- that we as armed citizens would be able, justified, or somehow required to match force-for-force when faced with a violent personal attack. (Or very strictly interpreting the "Force + 1" dictum.)
The doctrine of equal force, developed on a prototype of two males of equal size and strength, held that, if attacked without a deadly weapon, one could not respond with a deadly weapon.
Worth noting, however it would be wrong to read into this a requirement that one must submit to a beating, rather than prevent physical harm with the means at hand. Especially -- as others have noted -- in that a gun is involved even if your attacker hasn't discovered it
yet.
Getting beaten to death simply because you didn't have time to locate or access the next item of the "force + 1" ladder is not a requirement of the law.
Incapacitation and death (either immediate or eventual) can occur from a punch or a kick. Grappling, trading blows, and other such physical contact can only reduce your chances of surviving the encounter.
My "force continuum" might look something like this:
1) Apologize/diffuse/de-escalate.
2) Retreat
3) Warn
4) Show (maybe, under certain circumstances/jurisdictions)
5) Shoot
"Take a punch" or "engage in fisticuffs" doesn't enter into it, unless blindsided or sucker-punched in which case, retreat is still the preferred response.
Note one other important concern: You do NOT want to be in a position where you could be claimed to be a "mutual combatant." Getting into a fistfight may ruin your affirmative defense for drawing your weapon later. Testifying in court that you attempted to leave, attempted to de-escalate, and only drew your weapon and/or fired when you had no choice left but to be assaulted or shoot is a lot better than testifying that you traded punches for a while and then drew and shot the guy when it looked like you might lose the fistfight.
(...And it's a whole lot better than saying,
"Well, about the time he kicked my head into the curb for the third time, I saw a pretty white light and I just wandered on up to these here pearly gates. Now, which way do I go to pick up my halo and wings?..." 
