BBM
The problem is you are thinking about it, rather than actually experiencing someone stabbing you with a knife.
We get this a lot, people who just can't believe a healthy male could be stabbed to death by a girl (a GIRL!) simply because she's holding a knife and he's not.
Rather than think about it, here's a useful experiment which will require the following equipment:
1. A 1" paint brush
2. Paint
3. A 12 year old girl.
Have the 12 year old girl daub the paint brush with red paint. Try to really annoy the 12 year old girl. Then instruct the 12 year old girl to use the paint brush as if it were a knife.
Try to take the paint brush away from her.
What will happen is you will start to get paint on your hands. If you try to grab the paint brush, it will slid through your hands. Obviously it won't cut your hand down to the (very useful) tendons by trying to grab it, but you can note the theoretical damage. Once you get paint on your hands you are now no longer allowed to use your hands. Trying to grab the 12 year old girl in a bear hug will result in you getting paint on your chest, stomach, face etc. At which point you might think it's a good idea to start running so as to figure out how to stop getting paint on you. At which point you will have paint all over your back.
Keep in mind that an actual knife is harder to grab, hurts more and actually does damage.
Oh, and this isn't a facetious experiment. Having done a lot of survival and primitive skills stuff with people who have thought wrong about a lot of stuff, using dummy knives is the best way to teach people that what they think they will do isn't what's actually going to happen and disarming people with knives is the sort of thing that you see in movies which, like blowing up gas tanks, is a lot harder to do in reality.
As a general rule of thumb you should plan on the person having the weapon (yes, even a GIRL!) such as gun, knife, baseball bat, etc winning the conflict whereas the person without a weapon (even manly men) tends to lose.
Wet, naked, ambushed guys who have been socialized since they were zygotes not to hit girls (or anyone, for that matter) have a distinct disadvantage.