You awake in the morning on the beach, Katie in your arms. Memories of the night return and you smile. You lay there in the sun, enjoying the feel of your arms around her and your legs intertwined...
...wait, legs? You realize your tail is gone! The moon must no longer be full so you and Katie are no longer mermaids. You also realize while Katie's head is against your chest, you have less chest than you did the night before. Having returned to human form you've also male again.
You wake Katie from her slumber.
"Mmmmm...morning Bobby..." she yawns.
"We're not mermaids anymore" you point out.
You feel Katie's legs move against yours. "I guess not."
Katie turns to face you. "But now we have to figure out how to deal with you being a..."
She pauses, having realized you no longer have breasts. That and something is poking her in the leg.
"You're a guy again!" she says with surprise. "Well...interesting. I wonder if I'd been rescued by a merman instead of a mermaid I would change genders when I transform every month?"
"If I had to chose between us being both mermaids or both mermen once a month, I'd definitely prefer mermaids." With a grin you add "Especially after last night."
Katie smiles back. "Definitely."
"So are you in a hurry to get home?"
"Not really. Why?"
Without a word, you roll Katie onto her back, with you on top and slide glently into her. Her eyes go wide for a moment with surprise then she moans softly.
You've got your manhood back and a shared secret with your girlfriend. You've got a month before you have to worry about getting back to the water and a beach to yourselves...