If I may take a shot at this--- (NOTE: I rarely play Dark Legion and when I do it's usually Illian or Muaji-baby so I have no investment in the outcome of this argument...that being said...)
I think we're drifting a bit into semantics here. Let's work backwards --
To me, the Natural Attack SA seems to be a catch-all for bites, claws, fists, etc. While one could argue they are not literal weapons (i.e. gun, spear, etc), they are essentially just like combat weapons (i.e. a method by which you damage your opponent). Therefore they would be subject to the same restrictions penalties etc. as any other "weapon" regarding the Secondary Attack SA.
In regards to the Behemoth - again the rules are a guideline and they can not cover every variable and I think we can all agree that in UWZ, as great a system as it is, there are one or two contradictions (hence the FAQ).
While the rules state one can not use a pole arm and side arm at the same time - I don't think the four armed death machine Behemoth was who they had in mind when they wrote this one. I suspect it was geared towards the average 2 armed trooper ("Only 2 arms! Pathetic!" "How can you play a 12 string guitar?" Laughs the Behemoth).
Considering the Behemoth has four arms (whether he be 2x lefty or 2x righty) one would assume he would use two to wield his spear, and one to wield his side arm, suffering the same penalties for anyone firing a second weapon.
Now, according to the secondary attack rules - the Behemoth could use both his spear (primary weapon) and his handcannon (secondary weapon) in 1 AC. (Ouchy! I know my brotherhood army is squirming right now).
And I think this is where the problem may lie. To me the issue is should the Decimator be a "side arm" - i.e. usable in CC.
If yes - then, sorry guys, the way I would read it (even if staring down the muzzle), it would be usable.
Also - , well there's no real need for that natural fist attack now is there? When would you use it.
If no - then I agree that the secondary attack would instead be the fist.
I think the answer to this one may lie with the higher-ups sharing with us what their vision/intention was when desiging this big bad boy.
Hope I helped.
D