I mean, the point of a mermaid isn’t that it’s a sexy looking fish, it’s that other than its tail, it looks like beautiful human women. In some stories, sailors don’t even realize they’re taking to mermaids because their tails are underwater. Also, mermaids were generally considered bad omens or malicious entities, and when they did seduce humans, it was usually to kill them, so it’s not like the folklore was very pro-mermaid sex.