Eggslot
When a female human is transformed into a mermaid, her lower half is replaced with a fin, leaving no place for a vagina. This process results in only an eggslot in which mermaid eggs are birthed out of.
"Wait a second, do mermaids have vaginas?"
"Nah man they just have eggslots!"
"Nah man they just have eggslots!"