It was my turn to blog today at Cafe Otherworld and I wrote a post detailing my theories about why there are so few paranormal/urban fantasy books about mermaids when there are more wereolves and vampires than you ever want to read about. Turns out that for most of the people who have responded in the comments so far, it's an anatomy problem. How, exactly, do mermaids DO IT? Maybe I have a dirty mind, but I always figured mermaids wer like cetaceans and did it while gliding together in perfect harmony.
I also know that some animal experts don't think it's a great idea for humans to swim with dolphins because it arouses the dolphins and ... unforseen situations can occur. (Apparently, male dolphins are pretty horny guys and have often been observed "humping" inanimate objects. Although I don't suppose that's dry-humping.
It's not that I want to write mermaid/human porn, but honestly, I don't see the problem. Mermaids have been part of human culture since the ancient Assyrians. Something about the myth of the mer has captured human imagination. So...why are there so few stories about mermaids?