Katie lay across the desk, her magically enhanced breasts heaving with her still half-aroused breathing. She panted like she'd just run a marathon. She was naked, surrounded by the shredded remnants of her clothing.
"Hey Katie," I said. She turned her head.
"If I walk out of this room, that's it. Forever. You'll never get another chance with me. You'll go back to your husband, and go on being a schoolteacher, and your life will be exactly as it was before."
Katie nodded. "I can live with that."
"Except for one thing," I said. "We've already had sex. I've already been inside you. I never made you cum, so you aren't transformed, and I never loosed my seed into you, so you aren't bound to me. But we're connected, Katie. We always will be. There will always be a little piece of me inside you. For the rest of your life, I'll always be right there."
Katie shook her head, furrowing her brow. "What do you mean? I don't follow."
"If I walk out that door without you as part of my mermaid harem," I said, "the part of my spirit that rubbed against your spirit stays here. It can never be otherwise. Therefore, I place this curse upon you. Either you join my mermaid harem today, or you'll be just like you are today, forever."
"What do you mean, just like I am?"
"Always young, always beautiful. You'll stop aging the moment I step out of this room. You'll always have those huge breasts I gave you. You'll always be able to conceive a child, but childbirth will never ruin your body. Even if you live a thousand years and have a thousand babies, you'll always look and feel as young, as nubile, and as fertile as you are right now, lying on that desk."
Katie shrugged. I noticed she was trembling with adrenaline from my unfinished rape. "Always young?" she asked. "I don't see the down side."
"And I'll have complete control of your libido."
Now Katie actually paused. "What?"
"I will decide when and whether you do or do not get aroused by anyone or anything. If I say you're aroused, you'll need to have sex with whatever man is right there. Even if it's your own brother, even if it's one of your teenage students, the need for sex will be absolute, and can only be soothed by having a man cum inside you. And if I don't say you're aroused, you can't have sex, not with anyone, not even your own husband. Even if we never lay eyes on one another again, I'll maintain a telepathic bond, and I'll have complete control of your libido. Even if I die, my control will be passed onto my firstborn son, so you won't get out of it that way. If I can't have you, Katie, I'll still be the only one, Katie, who decides who, or even if, anyone ever has you again."
Katie looked worried. After enlarging her breasts, and almost turning her legs into a tail, she knew I really had the power to follow through on my threat. Katie shook her head at me. "Please, Jake. Please don't do this to me."
I reached out a hand and slowly, gently, stroked one of her magically enlarged breasts. It's your choice, Katie. Either I fuck you right here and now, and you relinquish your humanity and become a mermaid. Or I leave you, but I keep control of your libido. If you become a mermaid, you'll never see your husband again, and you'll have my babies and never love any man again but me, but you'll retain your free will. If you stay human, you can return to your life, but I'll partly control you."
I laid down on the desk next to Katie, my cock still out and engorged, and nuzzled her beautiful, magically enhanced breast. "What comes next is entirely up to you, Katie."