That is the meme, yes, but it's not the reality. Women are into horses because girls are into horses. Girls are into horses because they're considered 'pretty' animals and society considers horses a 'girly' thing. I'm not particularly interested enough in the subject to see where that comes from or when it got twisted from a manly thing to a girly thing, but outside of weird deviants it's not about sex. At least this explains why someone would come up with an insane conclusion like 'riding a horse is a sexually stimulating experience for a woman' that flies in the face of common sense, though. Going into the subject assuming 'women all want to have sex with horses' and trying to make up scientific reasons to explain this would lead to bizarre conclusions. It'd be like trying to come up with a scientific/biological conclusion to explain why all men are obsessed with the board game "Sorry!".