The manga quite obviously has a romance theme and it's about the two characters dealing with their feelings for each other. It's irrelevant whether or not something actually happens or if said feelings are reciprocated, and for the scanlator to use the " literary sense" argument is quite...
This was kinda dumb. Sango literally knows that a human-mermaid relationship is totally okay, she just heard that from the king himself a few chapters ago, so she had no reason to bail at the last second.
Good thing Himada was there to save the day.