The groin is the first valuable area in the line of fire? I think your instructor, while well-intended, is basing the arguments on a serious of assumptions, many of which may not be correct or applicable.
First, the instructor assumes the BG to be male, apparently. They aren't always male. If you shoot anyone in the groin area, it will hurt, but it is actually a SMALL target and you will probably miss the groin itself.
Second, the instructor assumes that you will be drawing from a side holster, not Mexican style, not SOB, not purse, and not fanny pack. The first valuable areas to come into the line of fire depend on the place or direction the gun is coming from. On a draw from a side holster, this would be the case, but not everyone uses side holsters.
Third, the instructor assumes a shot to the groin will produce a type of "stop shot" from what you described. One of the big arguments for not shooting people in the pelvis first (instead of COM) is that while the shot may reduce Bad Person mobility, it leaves the person fully able to shoot at you even though they may not be able to chase you. Additionally, there is virtually no chance that if you shoot someone in the groin that it will produce quick incapacitation, only mobility loss.
Fourth, the instructor assumes that you are a brilliant shot under a stress situation. The idea that you get a bonus shot to the crown of the head when the bad person bends over from the groin shot is rather naive. If you were good enough to shoot the groin in the first place, why not just direct your first shot to the eye area and produce a higher probability incapacitation shot (assuming you hit the eye area and the bullet travels into the brain). A head shot would be somewhat easier to make since the head is larger than the groin. As far as shooting the crown. The bone on the top of the head is much thicker than the bones of the face below the eye brows. A small caliber bullet may be deflected by the crown.
Fifth, the instructor seems to imply that a groin shot will stop the person. This seems to be based, in part, on the idea that pelvis shots stop people because a shot to pelvis will break it. This is a lot of wishful thinking. Assuming the pelvis is hit, it may simply be punctured, not broken, and hence the person will not be immobilized. They will be in pain and may limp, but not necessarily immobile.
Sixth, temperature can play a role as well, assuming the bad person is male. Many men, under cold conditions, have a condition where the penis and scrotum shrink up and seem to draw themselves to inside the body. This is a natural reaction to maintaining body heat for the reproductive parts. What does this mean? It means your target area just got significantly smaller than what you may have anticipated.
Seventh, if you miss the groin and shoot a little low, then you have likely totaly missed the bad person and wasted a shot. The beauty of COM shots is that they allow for the greatest margin of error, high, low, left, or right, while still producing a hit on the body. The groin shot does not.
Don't get me wrong here. Under stress, any shot to the body is better than a missed shot and a groin shot will get someone's attention (male or female), but then again, so will any shot to the body. However, I would not make the groin my primary target and by designating your first shot to the groin means that you have designated it as your primary target. After all, what if you ONLY GET ONE SHOT and it does not stop the bad person because it failed to connect with anything of importance in the small groin area? At that point, just click your heels together and start chanting, "I wish I were home, I wish I were home."
Sarcasm aside, I think your CHL instructor has some issues with groins that maybe need to be addressed by a professional...be it a shrink or a qualified tactical shooter.
Also, if your instructor spent 45 minutes on this topic, then you didn't get the information you should have gotten from the CHL class. You may wish to write DPS and tell them of your groin-topic experience because that does not appear to be standard DPS CHL instruction and it certainly is not good tactical instruction.