Horses had to adapt to an environment filled with predators, because of this, they ended up evolving to become quickshots, horse sex lasts for around 10 seconds on average. This way they barely had to waste time breeding, meaning they could go back to scout for said predators. Mares however, didn't evolve the same way as stallions, in fact, there's a really big difference when it comes to reproductive organs between the two. A stallion's penis is on average 18 inches long. A mare's vagina? 5 to 8 inches deep. This goes back to what I said before, the point of horse sex is to breed as efficiently and as quickly as possible.
The problem with this whole thing is that the stallion ends up satisfied, but the mare? Not by a long shot, mares are some of the most sexually repressed animals on the planet and the only reason they tolerate being jammed with a cock like that is because of their extremely powerful heats. The pussy lips of a mare are very sensitive from all of the nerve endings located there, so even a dick considered small by human standards would be enough to get the mare off. There's also the fact that a mare's vagina is really freaking tight but they have extreme muscle control over it, which is how they can accommodate a stallion's dick in the first place.
This means that a mare's vagina's circumference could shrink to about half the size of the average male's penis girth. However, size factor is also put into play. A mare's vagina is approximately 4x bigger than a human female's vagina. This means a mare's vagina is prone to excrete far more denser, high-temperature sexual fluids during intercourse. Additionally, because horse anatomy is modeled after natural evolution (and horses are enact quick-breeding sessions for maximum efficiency), a mare's vagina is capable of convulsing in certain patterns that "suck" a horse's penis to milk his semen as quickly as possible.
In short words, a human's dick would feel just right for a mare, and the fact that humans can last several minutes during sex as opposed to a measly 10 seconds, it's safe to say that a mare would in fact enjoy sex with humans quite a bit.
