Kevin could feel his legs merging into a tail as he made out with Gisela and Lilia in his hot tub. As the scales covered his tail, he felt his penis retract into a sheath. This struck Kevin as strange as he was so horny right now, but he of course knew pretty much nothing about mermaid physiology so maybe that was normal? Then he felt Lilia's hands on his chest and that definitely didn't feel normal. His chest had never been that sensitive to anyone's touch. And the way her hands grabbed his mounds of flesh...
Kevin pulled way from the two mermaids to examine his body. Something was wrong - he had breasts! He felt his crotch and found his penis wasn't simply sheathed. He felt something on his shoulders and found his hair had grown longer.
"I've turned into a mermaid!" Kevin gasped, his voice now feminine as well.
Lilia and Gisela turned to look at their new fellow mermaid.
"You make a pretty good mermaid" said Gisela. Lilia just smirked.
Kevin stared at her in disbelief. "You knew this would happen!" he exclaimed.
"I did" admitted Lilia. "Consider it payback for cheating on me with Gisela. And for cheating on Gisela with me."
"I'm sorry!" wailed Kevin. "Is this permanent?"
"Just like when I drink the serum, after 24 hours it will wear off and you'll return to normal...unless..."
Kevin gulped. "Unless what?"
Lilia slid up against Kevin.
"If you have an orgasm while you're mermaid you'll still go back to being human after 24 hours, but you'll be permanently female. You've been careless with your actions over the last several days, Kevin - or should I call you Kay? So be more careful, especially around us dangerous sirens."
Lilia pressed her lips to "Kay" and pressed her body against her boyfriend-turned-mermaid. Conflicting thoughts ran through Kevin's/Kay's mind. Kevin didn't want to be stuck as Kay, but Kay was still incredibly horny.