As you sat in the hot tub, you realized that your girlfriend’s naked body wasn’t interesting to you. She pretty, sure, but you just weren’t attracted. Which was weird. You *remembered* being attracted to her. The two of you had had sex plenty of times.
You yawned, and looked around. The people on the beach were pretty far away, and it didn’t look like anybody had noticed your indecency. There were a few dudes playing volleyball - they seemed pretty focused on the game. You found yourself enthralled by the sport, enjoying the movement of sand-dusted muscles and bouncing pectorals.
Your girlfriend cleared her throat, obviously vying for your attention.
She had slunk down in the tub, everything below her nose was under water. When you turned to look at her, she tilted her head up and spat at steam of water at you in a gross and kind of juvenile move, and then laughed. “It’s a saltwater hot tub!” She grinned. You gave her an unimpressed look.
She looked about ready to splash you when a look of concern and discomfort crossed her face.
“Maybe it was a bad idea to put hot tub water in my mouth. I feel kinda …weird” she said. Her stomach growled loudly, and a couple of strange lurching sounds cut through the awkward pause.
You watched detachedly as she began to tremble. Her breath was rapid, and she slid over to clutch the side of the hot tub, trying to stabilize herself.
She began to shake more violently, seemingly alternating between flexing and relaxing every muscle in her body. As she did this, the muscles began to gain bulk and definition. Your girlfriend had been in good shape, but never had she looked this strong. You watched in fascination as she let go of the wall of the hot tub and sprawled her arms wide - her chest looked different. Her once-ample boobs were receding before your eyes. As her pectoral muscles twitched, fat dwindled away. Once her chest was flat, her nipples shrunk too. Then, muscles began to clench and relax in an undulating wave, staring at her belly and riding up to her shoulders and neck. With each wave of engagement, muscle thickened and toned. Her shoulders and ribcage grew wider. Her abs became defined and strong. The cords of muscle in her neck engaged and she moaned, voice cracking as she did so. An Adam’s Apple began to gently poke out. Her jaw broadened and her browbone protruded subtly, eyebrows darkening and becoming just a touch more rugged.
You had never found your girlfriend so appealing.
She moaned again, noticeably deeper this time, and crossed her legs tightly and began to writhe, legs rubbing against one another and hips moving in a circular motion. You felt something brush against your feet, and you saw that your girlfriend’s feet had fused together and morphed into a long flat fin, and her legs were beginning to connect into a single tail, starting from the fin. Beautiful shimmering scales began to grow out of the skin once it fused
Your eyes followed her legs back up, and eventually ran into her larger and more masculine hand, touching her crotch. You watched with interest as your girlfriend massaged her clit. Now, you’d never had trouble finding that thing before, but there was no missing it now. It was engorged and increasingly phallic, swelling further with every stroke. She was soon able to take it in her hand and truly jerk it. You wanted to reach over and help, but it felt impolite to rob her of the pleasure of discovery. Soon, it felt somewhat strange to be calling her your girlfriend.
She(?) reached completion at about the same time as the fusion in her legs did. A sort of pocket formed and the now-flaccid dick retracted seamlessly into her tail.
Your girlfriend(?) sighed in satisfaction. A cluster of bubbles arose from freshly formed gills delicately lining the bottom edge of her(?) well-formed pectorals
“Well, maybe it wasn’t the worst idea after all. Your boner says you’re pretty happy with having an aquatic boyfriend…” he said with a mischievous smirk on his pretty face. His voice was wonderful, just on the low end of tenor, and so undeniably masculine.
You smiled, absolutely dumbstruck by the whole situation. You hadn’t been this turned on in a long time. It didn’t matter that you were suddenly gay and your girlfriend was suddenly a beautiful merman, it was already a great vacation.