Which is how I learned, without really intending it, that Pattinson has a new film called The Lighthouse , and that it involves human-on-mermaid sex. What should I know about mermaid reproductive anatomy to make it as enjoyable as possible for both of us? A few years ago, HelloGiggles admirably asked a marine biologist to come up with a mer-sex hypothesis grounded in models of fish reproduction. Others have suggested a different underwater comparison: Male dolphins have penises that fully retract into a small slit and when they emerge—oh god, they are monstrous and pink and I do not recommend a Google Images search.