You can ask the internet how mermaids have sex. If you do, you get people complaining irately about how the original Hans Christian Andersen illustrations have a tail starting below the bum, and then things got Disneyfied. Or a serious-sounding guy explaining
They can leave the water for certain limited periods of time, when this happens they assume human form, similar to the selkie. It is during these periods that they choose to mate, unsuspecting mates would not be able to tell them from anyone else.in such a forlorn way that I am CERTAIN that this happened to him.
But then you notice that The Mermaid Problem is literally a thing to the extent that Wikipedia has a quite long page of all the places it's appeared in literature and popular culture (Futurama and Red Dwarf are just two places that think reverse mermaids are more sensible, and Magritte painted one):
The blog Confused? Let's google it Has done the best work on this for hurried readers, and it turns out that answer has been in front of your eyes all this time, as I promised.
In ye olden days, mermaids looked like this:
(Ok, technically this is a melusine, I think, but let's for now assume that melusines are the basis for mermaid myths, since they look more like mermaids than stupid manatees.) Where have you seen one of these before? Oh yes! Here!
And we can all imagine how, well, like I said, that's your lot.
* For now. If you come to my much anticipated Kilburn storytelling session, probably be in January, who knows?**
** I know