Kasey pushed me gently down on the couch and kissed me again. I could feel her legs straddling me at first, but then they started to slide away. As we continued to make out on the couch, I could feel something new against my legs: a single, scaly appendage. At first it fell between my legs; where Kasey had started out straddling my legs, now my legs straddled her tail. Her tail rubbed against my crotch, making even more aroused than I already was. Kasey seemed to sense this as I felt her tail shift to one side. Though I could feel her tail moving, I couldn't see it and Kasey seemed to be in no hurry for us to go anywhere as our kissing continued.
Soon I felt the weight of her tail as it slid over one of my legs and off again against the back of the couch. Then her tail was pushing under one leg and then the other...and then wrapping around. My legs were pushed together and held there as Kasey's tail completely wrapped around my legs, but held me gently.
Once Kasey had my legs immobilized in her coils, she pulled her lips away from mine and flicked her tongue at me. It still looked mostly human, except it was longer and forked at the end. Her upper body seemed unchanged. As she pushed herself up and arched her back, I looked over her body. I could make out a few scales on her sides and a belly plates below her stomach. She had changed herself to a naga; a snake from the waist down.
"How do you like my new form?" she asked.
"It's pretty impressive" I admitted.
"Then join me."
"What?"
"Become a naga like me."
"I've never thought about being a snake before - not even part-snake."
"You should; it feels wonderful. And remember how I said I REALLY love snakes?"
"Yes..."
Kasey just smiled at me with a seductive grin.
"...oh!"
"Yessss" she hissed. "Exactly."