I asked my son what he thought. He said he would ride every day if the horse was practical. My son and husband don't ride, but I notice they feed the horses treats and talk to them often.
I read a book once that answered that question a lot, and it ended up being an instance of similar social behavior (I'm sure there are more reasons, this is just one). In a social situation, a group of girls will usually work out relationships with one another before competing for a position of power. It's the same with horses. When a horse is put in a paddock with a human, they will bond simply from a desire to relate to another being. With boys and men, it's more of an establishment of power. Thee's also the personal space. What I mean by that is the way you approach a horse; from the side versus simply walking straight up to them. This one is one I thought of on my own. Two boys who don't know each other will walk straight up to each other and be comfortable, but a girl usually prefers to either keep more space or be approached from the side. It's amazing the similarities you can find.
I think boys wanna act tough, so they think that horses won't make them tough. They are not sensetive enough to try to love a horse and they don't want to be known as "The horse boy" by all the other boys.
I know I love my horses - and have since just a little bitty girl, because I do imagine myself free in the wide open, able to escape danger on the back of my powerful steed, but mostly because I can't MAKE my horse DO anything except what they CHOOSE to do because i have a PARTNERSHIP with them. They do what I ask because they respect my leadership, and trust my motives. There are few other places in my day to day life that has such a pure and dependable relationship. And to add to all of that, they are art in living form!
I also think that horses are commonly accepted by girls, but the average boy (around here, at least) who loves and rides horses will generally be accepted more by girls than other boys. With girls, it's 'cool' to ride. Boys? Not so much..
Why do I love horses? I will never know, but it's been a lifetime passion of mine. Horses are swift and agile as a deer, yet strong and sturdy as an ox, and loving and friendly as a dog. They have been created with the best attributes of animals. They are amazing!
Girls want to nurture their horses yet while riding, fly like the wind. They know their horses can be loved and they are loved back. But, the two, loving your horse and flying like the wind, fulfills that lonely place in your heart. The sense of belonging to something.
I believe that your last comment about girls having more of a nurturing instinct is quite true. I also believe that horses are and extension of girls playing with dolls. Dressing them brushing their hair etc....they do the same thing with horses.