The question is even how they mate. Please note the way fishes mate is completely different from how humans do it. Fishes unlike humans, don't even touch each other... The females lay eggs and the males spray their sperm onto them. Fishes are completely different from mammals on this point (the shark maybe as a notable exception as sharks can get pregnant).
Now mermaids are beings from folklore and their reproduction stories are not mentioned in folklore. The fact that their intelligence is similar to humans does not mean their anatomy characteristics are the same. Fantasy writers have come up with people who look like humans, but who are not born out of pregnancy, but who hatch from eggs. Since folklore does not discuss the details of how mermaids come to be, I guess writers who come up with stories about them do therefore have the full freedom to come up with their own thoughts on this one.
One nice detail... Mermaids, and their male counterparts, mermen, are often depicted having a navel. This would indicate they are born from a womb and not from laid eggs, since creatures born from laid eggs do normally not have a navel. After all, the navel has no more medical function after birth, as it's merely the scar left when the connection with the mother body (for providing food and oxygen) is severed... When you are inside an egg you don't have a physical connection with your mother. However mermaids come in all shapes and sizes, and there are also the kind where the fish part reaches up to the shoulders only leaving human arms and a human head, and then things are completely different, and then the navel is missing and then the egg story becomes more into play.
And then there's the kind of mermaid is in the romantic comedy "Splash" in which Tom Hanks falls in love with a mermaid. The mermaid here only has her fish part when in salt water. Does that mean that when not in salt water, her reproduction organs work the same as on humans?
Well since mermaids are not real, and come in many shapes and sizes, I guess anything is possible depending on the sage and the writer of that saga. For me as a fantasy writer, good news 😜😜