You moan as you push into her. She is warm and moist and tight, and getting tighter as you proceed. You enthusiastically pump away at her. You can hear her moaning in pleasure and her tail swishes around managing to hit you in the face a couple of times. You didn't think you could have gotten any harder, but it seems you are. You barely realize that what's actually happening is that you're getting larger, not harder. For that matter, you can feel an odd feeling in your tailbone. Like something is pushing out from it, forming a swelling back there. You feel a large lump grow out. Your logical mind reasserts itself, telling you to get out of there. You try to pull out but find that she's so tight around you that pulling out is difficult.
"Don't stop, baby!" the woman cries. You try, despite her protestations, but something seems to slip. You fall forward somehow and your hands touch the floor beneath you. Physically this shouldn't be possible, but a quick stalk of your legs tells you that your legs have grown tall and equine. A tall flomps out from the growth on your back and hangs beneath a rear-end that has grown much larger. Now pulling out is quiet impossible. Your hands clench up against your will and your fingers thicken, and engulf each other. They then turn hard and black as brown horse hide crawls up your arms, turning them into horse legs.
Your face begins stretching out before you as well and hair trickles down an neck that is ever lengthening. You try to speak but all that escapes from your throat is a frightened whinny. You watch as the brown hide begins forming over your changing nostrils and lengthened head. You realize as the last stages of your transformation come to a close that the woman you are with is also changing. Already her legs and rump have become equine in nature. She moans with pleasure as she changes and that moan evolves into a whinny as her changes complete.
It occurs to you that you could now easily pull out of the mare. But you're still not finished and her scent is so intoxicating that it's difficult to think of anything else but continuing to hump this beautiful creature before you. You catch yourself humping without even realizing. You're close to climax, but something in the very back of your mind says that would be a very bad idea.