You soon feel your legs again, and find yourself once again wearing a tail instead of having a tail. But nothing everything is back to the way it was before you put on the tail. You unzip the tail and pull it off to confirm.
"I'm not a mermaid anymore, but I'm still a woman!" you say. Your voice, like your body, is still undeniably feminine.
"I guess it doesn't work quite like I expected" admitted your girlfriend "but like I said before, it was meant to turn me into a mermaid, not you. Was it as sensitive as it seemed?"
"It was amazing" you admit.
Your girlfriend grabs the tail and begins sliding into it. "Good, then you can return the favor because I'm horny as hell."
She slid into the costume as quick as she could and zipped it up and began rubbing her crotch. Her transformation began immediately. You watched eagerly. The thought of going down on your girlfriend as a mermaid made you wet.
"Something doesn't feel right..." said your girlfriend. There was clear alarm in her voice, but also something else. It sounded...deeper?
"Oh no..." she said. She wasn't rubbing her crotch anymore, but it looked like she was holding something in her hand. Wait, was her hair getting shorter? And her chest smaller?
"This stupid costume really doesn't work the way it should." she exclaimed, her voice now noticeably deeper. "It doesn't turn you into a mermaid regardless of your normal gender; it turns you into a mer of the opposite gender!"
Even before she let go of what was in her hand your realized what it was: an erect penis. The tail costume turned you into a mermaid that reverted to a woman, and now your girlfriend was a merman with an unmistakably male body. You were a man in a woman's body, but your body still had hormones and the merman before you was still your lover. You were still horny, perhaps now more than ever. Without hesitation, you leaned over and took the mer-cock in your mouth.