The “reach” issue in general is my main reason why this is a bad idea (er, well maybe my third main reason, after 1) don’t fight bears in the first place, and 2) if you must fight bears, use an appropriate tactic):
If the bear is on all fours and facing you, your leg has no way to get to your intended target.
If the bear is standing up on two legs for some reason, bears tend to be much bigger than humans. So you still are unlikely to have the time and geometry and speed to get in the kick before getting shredded, unless it’s really a man in a bear suit standing around waiting for you to do so, as seen in the comedy video above.
Bears are fast. If you are going to get close enough to kick it down low, you may get killed before you get a chance.
The bear is huge and you probably don’t have great familiarity with where would really work to kick, and you are probably not going to be thinking clearly while facing a bear, and if you don’t completely incapacitate the bear immediately, you will be putting yourself in a great position for it to do whatever it likes to you.
Even if you did stun a standing bear with such a kick, it might fall over on you. It might also be really angry with you once it recovered.
Bears might also just be more resilient to this kind of impact than humans, so even if everything goes well, it might not incapacitate it.
That whole thing about bear testicle retraction sounds like a very good point too.
It will almost never be worth taking any chances that you might get this opportunity, even if it did have a chance it would work, unless you have a fatal diagnosis and your life goal is to be remembered as the guy who kicked a bear in the scrotum, or died trying. Or if your goal is just a quick death for yourself.