You may have noticed what most experienced trainers say, the absolute worst person in the world to train a woman in gun handling is their husband or boyfriend. The truth is that even if all men WERE born with the I can shoot gene, many of them stink as teachers, and a close personal relationship with the student will only make it worse.
You may also have heard the phrase "it's one thing to know, it's another to teach". It sorta goes in hand with "those that can't, teach" in that being able to do something well isn't a requirement in order to teach it well. Coaches for female olympic gymnasts are a great example of this.
There are three main ways people learn--by reading/hearing, watching or doing. Different people respond (or learn better) with one method compared to another. Thus just because you learned to be a decent shot by reading about technique doesn't mean everyone will be able to learn that skill the same way. A good instructor can teach using all three methods. A great instructor picks up which method works best for a given student and tailors their training appropriately.