Well that's the only way for her to get life force to live as a human.. So not exactly per say "slutty".
Do you remember the title? It says EROTIC Fary Talse. Don't complain.
but she needs the sex for the life force in order to live
Wouldn't be much of an erotic fairy tale if she didn't have sex this often.
This is one slutty mermaid...always needed to be fuck...i mean she get fuck every 30 seconds (not manga time-wise).