Mermaids are actually fictional characters, meaning they are not real. If mermaids were in fact real, most likely they would not be able to mate, due to the way there bodies are made.
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A mermaid is a mythological creature, so they only exist and mate in our imagination. However, male sailors thought out history have thought about having sex with them. You can find more information here: http://en.wikipedia.org/wiki/Mermaid