A horses cock is usually somewhere between two to five feet long when erect. The length to a womans vital organs is signifigantly less than this. The Width is also usually one to three or even as many as four inches wide. This math does not work out. If a horse, which is alot stronger than you are, shoves it's erect member into you too hard, it will literally pierce your vital organs.
If a horse isn't aroused for whatever reason and decides to rest, IT WILL SIT DOWN ON YOU BECAUSE YOU ARE UNDER IT.
If the horse becomes panicked, it will run, because the harness holds you so close to the horses body, it will kick you with it's shins in your legs and head with great force.
In conclusion, do not have sex with horses unless you are a professional, have the proper training, don't care what other people will think if they find out, and don't mind the possibility of extreme bodily injury and/or death, and breaking the law. :]