Futurama had it the females laid eggs like fish, and the male's wanked on the eggs.
In truth, being mammals, merfolk reproduction would be similar to that of the whale or porpoise. Himpa bumpa = live birth.
As they are a hybrid, it is possible that they mate like the platypus, humpa bumpa, but lay eggs.
I'm ashamed to say I know these theories.
Caption this Meme
