"Look on the bright side: you get the chance to have orgasms!" said Amy. "Besides, we had sex last night when I was a mermaid without you turning into a mermaid. You only transformed after we had sex in the water."
"The next time I find a genie I'm going to say no thanks."
You realized it was a good idea for you to leave before Amy's parents got home. You go dressed, said your goodbyes (with a long kiss), and agreed the two of you needed to have a talk about the strange turn your relationship had now taken.
You went home and soon found yourself occupied by chores and other mundane tasks. But that night you had a very vivid dream that you were having a very heavy makeout session with Amy, except you were both naked and you had a woman's body. You woke up the next morning drenched with sweat.
Slowly making your way out of bed, you headed to the bathroom for a badly needed shower. The warm water felt wonderful on your skin and you felt very relaxed. At least until your legs began to feel weak. You fell back first against the tile and slowly slid down the wall as you lost the strength to stand. You realized what was happening with a mix of fear and excitement. Life had definitely gotten a lot more complicated. Soon you were laying in the tub, your tailfin draped over the side, your ample chest heaving.