The lust is too much for you to bare. You can barely think of anything else. You tell yourself that you'll just do it a little, just enough to get it off your mind. You want finish. That's what you tell yourself. You rear up and push your fully aroused self deep inside the mare. The pleasure is unlike anything you've ever felt before. Perhaps it's better, or perhaps it's just that you've never wanted it more, but you suddenly loose all sense of yourself and your plight. You let out sound wholly unfamiliar to you and your animal instincts seem to take over. You push yourself deep inside this mares heat with lustful abandon. A laugh from behind you is all that snaps you back to the reality of your situation.
You realize how close you were to giving in, to releasing your seed into the mare and being caught in this horse body forever. You curse yourself for being so stupid. You try to retract yourself from her. It doesn't work. You try again, but the soft moist, wet walls tighten around you. You try once more before you see the mare turn to look at you as though she knew what was going through your head.
"Oh no, my boy, that just won't do. You haven't finished yet." you hear the woman say. You try a few more times, it still doesn't work, but it feels so good that your animal instinct take over and you thrust and pull thrust for pleasure. You return to your heady rhythm.
"Thaaat's right, and my magic spell will keep you from escaping until you've cursed yourself forever. So let yourself love it, let yourself want it." she says "Give your humanity away willingly." As she says this you feel your inhibitions falling away. And as she speaks, you find yourself wanting the curse. You know you're trapped now and hump away with no fears or inhibitions. Somehow this makes the act feel that much better. Soon with another strange cry you release your seed inside the mare. In the long awaited moment or release your feel your muscles tingle and somehow your form feels so natural that you'd swear you were born with it. You finally withdrawal from the mare and walk around.
"Very good." the sorceress says.