SPOILERS AHEAD!!!!!!
So, to answer CaptainWacky's question further, I am going to assume that all female and male parts are functioning while the mermaids are on land. Ryn, the murrmaid was shown entering into the restroom after a night of sleep. I am sure it wasn't to apply make up.
The last episode on Siren left us with Ben, Ryn and Maddie kissing. This episode begins with all of them in bed, spooning, in shorts and tank tops or t-shirts. And when they wake up Ryn asks why they all stopped kissing. Ben and Maddie explained human love making to Ryn at this point-who only knows about mating as a mermaid where the male gets bitten and often dies.
Then Ryn tells Ben and Maddie that she wants to make human love to them. So maybe that will happen sometime in the future if they don't all die first.