As you get closer to the mare ready to mount, you have a quick moment of clarity and realize the risks in front of you. If you follow the beastly urges that your body so desires, you'll be a beast forever. You start to panic as the realization sets in.
In your panic, you begin to jitter. Red takes notice of this and steps closer to you to put her hand on your shoulder in an attempt to calm you down. In your panicked mind, this startles you causing you to jump back.
While doing so you knock Red over onto her butt in front of you. You lower your head down to her and again are greeted by the musky smell of her womanhood. This causes your mind to fill further with lust as you lower your muzzle even lower on her body. Finally, you put your muzzle between her legs again, taking in deep breaths of the musky smell of her womanhood. This is causing your mind to fill even more with lusty thoughts, further spurred on by your overfull equine balls churning with desire, full of both your human and now equine seeds. You start to leak precum from your throbbing horse cock.