I should know better, but, do you have a cite or something? I mean, MY goddam sack is easily hurt. Where do you think those “normal or semi-normal reactions” come from, book learning? That’s what the cup is for in the first place.
Have or show some "sack" as statement of testicular fortitude. When did this become a common saying?
How old are you? How many times has your easily hurt sack actually been hurt? Do you wear a cup all the time? Why not? I will tell you why not- because you just are not going to get hit there, and if you do, the blow is likely to be glancing and not damaging.
I don’t have time now, but I am sure a little google-fu will find endless amounts of respectable martial arts sites that explain why not to waste time trying to kick someone in the nuts. It is hard to get there, it is hard to score a direct hit, and it is too risky to get caught right there.
Why? Because, as you intimated, instinct kicks in (no pun intended), the fleshy think in front of the sack is not nearly as sensitive to pain, and the legs are on either side. The little spot is just too small and well hidden and easily moved to depend on hitting.
Plus men are generally very aware of the pain a hit to said sack can cause, and go to great lengths to protect it. I’ve never been in a fight myself, but I’ve heard from multiple sources that it’s much more difficult to score a square kick to the balls than people think.
Sack as a euphemism for courage? I can’t be positive, but I’d bet that I was hearing that as far back as the 80s. I’m 99% sure I heard it in the early 90s. I can even picture a particular coworker saying it.
I’ve never heard this phrase and, since it sounds ridiculously pretentious, I’d put the utterer of this idiocy on my list of those who are beneath my future notice.
Alternative morph: “satchel”. I have a friend who uses this term constantly.
Sorry to dig up up a zombie(by the railroad tracks 
), but I just saw the movie where I first saw ‘sack’ in that context. Why I thought it was Richie is a whole different mystery, but Stand by Me, which remote info says is an '86 movie, just clicked as my memory as I just now saw the scene where I first heard ‘sack’ ;Wesley Crusher threatens to shoot Jack Bauer.