I don't know but it seems to be! I stopped riding horses because I don't like bossing an animal around. That's also why I have a cat (ok, 3 cats) rather than a dog.
Besides, a bicycle is a lot more practical.
However, when I was 10 I went on a camp where you rode horses twice a day for 45 minutes, including bareback at the end. Later I went riding twice more (once in Italy, was very nice), but then I quit, for the aforementioned reasons.
I've never met another guy who could ride horses, I think. Part of the big thing of riding a horse is also taking care of the horse, brushing it, etc. That's traditionally been considered more something for the ladies (witness the my little pony marketing).
This world is incredibly sexist, and it's nowhere as obvious as when you look at what kids are confronted with when they're little. My parents were very different in this respect, compared to just about all other parents. I did all sorts of things as a kid, from cooking class to go-karting and all sorts of stuff in-between, and there were very clearly stuff that people made girls do, and stuff that people made boys do. There's an incredible amount of examples I can come up with and only a few show some kind of balance in the sexes. And almost all of them can be traced back to 'genderism' in parents and the rest of the people that kids deal with.
Anyway, straying off-topic.