You can hear beautiful, almost ethereal singing that echoes in your head. You continue to focus on the mermaid's beautiful face until you notice the strange way her hair floated freely around her head. A quick look around, and you realize you're pinned down by the mermaid; under water! The first thing that went through your mind was that they were going to drown and eat you, just like in the more horrifying tales of merfolk. Those thoughts quickly dissipated when you realize, you were breathing comfortably; though, the brief moment of terror was enough to make your little man go limp.
Still, the fact you're breathing normally underwater was unsettling in and of itself. "W-what's going on!?" You shout, only to gasp as you hear a decidedly more feminine version of your voice.
"Our Sorority could always use more sisters." The mermaid says as she runs one hand along your smoothed out face and through your ever lengthening hair before placing it on one of the mounds swelling on your chest. Meanwhile, you feel the grip of her other hand on your arm change as its diameter shrinks.
"Sorority?" You figured that's what a group of mermaids called themselves; like the sorority houses at a college. "Wait, SISTER!?" You shout as you feel your waist shrink and your hips widen. It was then, you realized the ethereal singing was from the other mermaids, circling overhead. Somehow, you know it's some kind of spell. "N-NO! I don't want to be a girl!" You struggle to free yourself from her grip, but the mermaid is surprisingly strong.
"Shhhhh..." she whispers in your ear, "...just relax, and soon you will sprout a beautiful tail; then, your new life as a mermaid can begin."
You re-double your efforts to break free as you feel your manhood shrink away, and a pink slit forming in its place. "Please, no- I don't want to be a mermaid!" As your legs loose their hair and they, as well as your feet slim down, you manage to wrest yourself from the mermaid's grip. You kick off the ocean floor and surface. If you can get away in time, maybe you can at least save your legs.