Have to admit I've always wondered how they had sex
Hehe, me too! I found this, though, so this would be what I'd go with.
' Mermaids, like dragons, can shift their skin to cover more or less of their legs (and what lies between them) and change the colour accordingly. Like hair, eye and skin colour, a mermaidís tail colour is determined by genetics. She can close her gills to appear human, when she breathes air on land.
Now, having chosen to permit her tail to part, her gills to close and her skin colour to normalise, a mermaid looks like any normal human.
Essentially, she is as capable as any human woman, though with her high level of fitness and flexibility from swimming all her life, perhaps a little more capable and with more stamina.'