"I fantasized about being a mermaid, but I didn't want to leave the land world behind forever" explained Amy. "So that's why I can shift between forms, although not really with the control I might have expected."
"But why I am growing a tail now too?" you asked.
"You didn't want your wish to hurt our relationship, and I didn't fantasize about being a mermaid who would leave her lover behind on land. I wanted the power to transform my lover so they could join me in the sea as well. The good news is your change will be just as semi-permanent as mine."
You could feel your legs melding together and soon you had a flesh-colored tail as well, a merman to match your mermaid girlfriend. Exploring your body you found your balls were now internal, and it seemed like your penis would retract into a sheath when not erect. However, in the wake of your transformation, it was quite erect and perhaps a little larger than before. Amy ran her hand down your mer-cock and smiled.
"That wish you made may be the best thing that's ever happened to us."