Ah, yes, the
Mermaid problem - Wikipedia, the free encyclopedia
I'm going with mammalian sex, pregnancy, and live birth. As that works much better than dead birth for continuing a species.
Male genitalia is internal, male gonads presumably in the same position as female gonads and the penis only leaving the when urination or the sex act occurs.
As far as mer-human reproduction, I can't figure out how their DNA could be compatible.
