Cum and you're done!! The Pet Feminization Curse

The hero invades the demon lord's castle, determined to rid the world of its occupant. But it turns out the demon lord is a powerful succubus who the hero has very little chance of defeating. Instead, the succubus places a curse on the hero that turns him more and more into a girl every time he cums. On top of that, she places a charm on him that makes him super horny. And as a cherry on top, should the curse progress as far as changing the hero's genitals, the succubus has another ace up her sleeves. Or rather, up her panties: she's a futanari.