Fact is, fence is more suggestion then physical barrier. They want out, their going to get out. That's why electric works. They learn.
I agree with this, and our old saying is that with horses being typically very food motivated, 95% of 'fencing' is keeping something to eat (grass or free choice hay) available all the time.
All our mares grew up with electric fencing, mind it without problem, and typically use only one strand of white polyrope, 3' off the ground, posts every 30'. Many folks are amazed, but we have never had a horse escape from this, it's very inexpensive and quick to put up, change, repair, etc.
In areas that have foals, we add a second strand about 1' off the ground, and on one side of a pasture that is next to a corn field, we have a third strand at about 4' for the deer.