I have no doubt some prefer one sex to the other.
I worked for 9 years for a male trainer, we definitely had some horses that went better for him or our male amateurs. And there were some that preferred me or our lady amateurs. Sometimes it was really subtle…just a bit of resistance or tension. Sometimes it was quite dramatic (like the one mare who about broke my boss’ nose whenever he tried to ride her, but she got along great with me and her owner…which is funny, as we often found that most mares went really well for the boss).
We had one little kids horse who, I’m SURE, must have had the tar beat out of him by a man, as he was just barely tolerant of even being led by the boss or the father of the kid who owned him (he dumped the dad- a pretty solid rider- and neither of them could bridle him. I always had to do it). He was a peach for me and the other ladies, and a SAINT with the kids. He had a lot of funny quirky stuff about him that made him bit of a PITA, but he was such a little doll, so cute with the kids, that the grown ups in his life just put up with him regarding some stuff.
We also had a horse who, while he didn’t so much as prefer women (he went on to compete pretty successfully with another male pro), did go a lot better for both the boss and his male amateur owner if I rode him at least once a week and gave him what we started calling his “estrogen fix.” He could be a tough horse who wouldn’t back down from a fight. If he put his mind against doing something, he would bar room brawl with you and escalate things REALLY fast. And he was the type to pick a new thing to dislike each week. I refused to fall for his tactics and would bore him into thinking my way, which, for whatever reason, was not something the guys would do. So, I got on him at least once a week, we’d have a pleasant little ride until he’d object to whatever his objection was that week, and then I would just sweet talk and bore him until he found it far more interesting to do that objectionable thing. Horses are silly. 