After having done some research, it seems that there are four different options, when it comes to thinking about how mermaid reproduction is possible:
1. Split tail, with genitalia in-between. Like the mermaid on the Starbucks logo, this involves mermaid with split tails, where their legs would have been.
2. In the 1800s, mermaids were drawn with the fin beginning below the genitalia.
3. If the mermaids are created as more fish-like, they'd have a slit in the fin, where the mermaid would expel eggs, and the male would then expel fluid to fertilize them.
4. Mermaids would mate like dolphins, with their genitalia tucked in their fin, accessed through a slit. This seems to be the most acceptable form to modern readers.
So there we have it! There is no consensus on this issue. Just pick one you like the best and run with it!