Lydia decieded that as much as she loved being a mermaid being one permanently would cause too many problems. Besides, the way to transform back...they had both waited, much longer than most teens in a relationship did. She was ready and it was clear her boyfriend was too.
They could go upstairs, back to the privacy of her bedroom. But her parents wouldn't be home for awhile and the fence around their yard was high enough to ensure privacy. The pool was tempting but she didn't want to risk drowning her boyfriend. And the rain was so inviting...
Lydia leaned in and kissed you deeply then broke it off to turn and dive into the pool. You watched her gracefully move under the water to the other side of the pool where she pulled herself out on the opposite side of the pool and motioned for you to join her.
You walk around the side of the pool to where Lydia is, now laying on her back in the rain with her tailfin dangling in the pool. You lay down next to her, not sure if she is still decieding and not wanting to push her. She pulls you on top of her and into another kiss, this one full of passion and desire. You awkwardly struggle to remove your shorts which are no longer needed.
Between kisses she says "I don't want you to think I am only doing this to be human again."
"I don't want you to think I am only doing this because you're a mermaid" you reply. Corny perhaps, but truthful.
You can smell the chlorine in her hair and the scent of the rain on the air. You straddle her and your fingers trance her stomach and the yellow lines of her suit/body as Lydia squirms with pleasure. She guides you in then bends her ever flexiable tail to your back to to guide your rythym. The rain falls all around you, running down your bodies.
After you both climax, her tail falls away to the concrete. You pull out and lay next to her, both of you exhausted by the love making.
The rain is finished too. You look at Lydia and she her smiling. You watch her, waiting for her to transform from mermaid back to human.