First, almost all western saddles will put you behind the horse's CG at speed. It takes a fair bit of forward leaning to get the rider's CG in the same spot as the horses, because the balance point of a western saddle is further back than with an Aussie or English saddle. I LIKE riding forward at a canter or gallop in a western saddle, but it means my shoulders are also forward. Well forward.
Although this guy is campdrafting, it illustrates riding forward in an Australian saddle, which is already more forward than most western saddles:
"One can be with the motion and the only way to do that is with a balanced seat where the heels and the CG of the body happen to be in vertical alignment. For flatwork, that means shoulder, hips, and heels in a vertical line."
CG and the stirrups ought to be pretty well aligned, but your CG is not determined by having a vertical line to heels, hips, shoulder. The guy in the photo above almost certainly has his CG with the horse's.
For WALKING, it is true. For a collected horse, it is pretty true. For an extended horse, trying to cover ground, it is not true. That is why you will never see a jockey riding with "shoulder, hips, and heels in a vertical line". In any forward seat, at speed, you do not have "shoulder, hips, and heels in a vertical line".
Being 'behind the horse' means your CG (center of gravity) is behind the horse's CG, which is roughly at the heart girth, but it varies with what the horse is doing. If your horse needs to make sudden stops, being behind the horse is GOOD, just as leaning back is good if you are standing on a bus that stops suddenly.
None of this applies much to the OP question. The OP is obviously not asking if an experienced rider has a problem while riding in ANY of the riding styles. You will never see a top dressage rider get off his horse and grab his balls - nor a top cutter, roper, or jumper.
Most of the concern is based on ignorance. Most men know how to ride a bicycle, and they don't grab their balls getting off a 10-speed either. It really shouldn't take a man more than about 5-10 seconds to find a position in a saddle that doesn't crush his nuts.
My advice was geared to a beginner rider, since experienced riders already know the answer. A mild chair seat (heels aligned with the belt buckle) and carrying some weight on the thighs and in the stirrups will help a first time male rider stay comfortable. And any male will sometimes do something that will feel a bit uncomfortable, but being smarter than a rock, he will figure out to adjust and not repeat.
If he enjoys riding, he may branch into jumping, dressage, roping, or trail riding to whatever extent he wants. When he does, he'll adjust his style of riding and, again, quickly figure out if something is causing a problem. It simply isn't that hard.
I have never boasted about my riding, but for reference, here is what I mean by a mild chair seat. And yes, my CG is behind the horse's at that point (ie, behind the base of the withers):
"The location of the horse's center of balance depends on a combination of speed and degree of collection. For a standing or quietly walking horse, it is slightly behind the heart girth and below the withers. If a horse is moving at a trot or canter, the center of balance shifts slightly forward, and it moves even more forward when the horse is galloping or jumping. If a horse is highly collected, the center of balance will be farther back, regardless of gait, than if the horse is in an extended frame."Center of balance (horse) - Wikipedia, the free encyclopedia
The article below is from the Department of Animal Sciences, University of Missouri-Columbia:
"The center of balance of a horse standing or walking freely lies directly over a point a few inches behind the withers. Selecting a Saddle
As the horse moves forward at speed, the point of balance moves forward. Jockeys provide a good example of weight well forward on the shoulder, permitting full potential performance of the horse. Even pleasure riders find that "getting forward" is not only comfortable for themselves, but it also seems to allow freer movement of the horse."