Never one to let a girl down you happily oblige. You stand behind her, and guide your shaft to the opening of her hungry snatch. Teasing her a little your begin to rub your dick around her lips. "Stop teasing! Put it in!" She cried as she bucked her hips back forcing your dick inside of her. She struggles to find her breath as you slide into her inch by inch, instead resorting to making gurgling "Ghhuaa" Noises with a few brays mixed in instead. As your pelvis collides with her ass you begin to draw back, eventually finding a rhythm of in and out that you find the most enjoyable. Jen comes to match you, both of you now in sync.
You can feel the final changes taking there grip as you fuck Jen. Your hands fuse together forming hooves, like your feet before them, and your skeletal structure re-arranges itself, forcing you down onto four legs. Jen's body begins to swell and expand, absorbing her breasts into muscle mass. Her hands having been hooves for a while now, easily integrate into her reforming legs, eventually snapping into place with a large crack, she now found herself supported by four strong Jennet legs. You grunt deeply with each thrust while Jen gace out higher pitched wines "Nghh... Yeeeah! Yeeeeah-Haaw!" She cried as you slide into her, over and over again. You feel your fur grow in, becoming a full coat, and looking down at Jen you can see she is experiencing a similer feat.
Jen and yourself were fast approaching the ends of your transformations as well as your sexual encounter. Jen felt herself fast approaching orgasm, as did you. "I... I'm.... Cummming!" She yelled ecstatically. "I'm Cummmming! Heee Hawwww!' The insides of her vagina clamped down hard as she came, her liquid spurting quickly from the sides of her pussy. This send's you over the edge as well as you pull Jen in tight and explode deep within her womb filling her up completely. You bray triumphantly "Heee Haaaaww!" Now a complete donkey. Your orgasms completed the transformation, leaving behind all human thought and reason, with both of your faces pushing out and forming complete donkey muzzles in the last seconds of your embrace.