"BASTARDS!" The warlock screamed at the top of his lungs. "I've tried again, and again, and again, to create transformation trinkets that didn't devolve into screwing each other like dumb animals! But no! No matter what, that's what it always comes back to! Sex, sex, and more sex! Well, if that's what you all want so badly, then so be it! Be animals to each other! Be disposable sex objects to each other! I GIVE IT ALL TO YOU! This is my last spell, my last curse! I offer myself as sacrifice to this spell! You're all doomed to the paradise of your own invention!"
The Warlock in his magic shop's basement stabbed himself, his body erupted in green flames. His body was totally vaporized, his soul was unmade by the fires of Hell itself, even the memory of him would fade into nothingness, all of it had been given instead to his final curse, to give it life. Without shape, without substance, without mass, without color, without sound, invisible to everything, only visible in how it would affect the world. And the world was now doomed. Once the curse completely ensnared all humans on the planet, every day would be the same as the last in an endless cycle or orgies, until the sun devoured the Earth billions of years later.
These things would be immortal, not need to eat or sleep, and every thought, idea, and action would be driven purely by the need to have sex... and every single one of them would be completely barren. There would no future generation. They'd have the world they'd foolishly wanted. Every human would age, or de-age, to their exact physical prime, there would be no children in the world ever again. And, as a final flavor to the joke, people would become anthros...
The animals of the wilds on the other hand would be unchanged. The warlock had had no grudges against them, they did simply as their instincts commanded them, but even they wouldn't kill unless hungry. They would go about their days, unaltered, consistent, no more giving a damn about these weird two legged things that did nothing but mate than they had before those bizarre hairless apes.
And so the cures began to spread...