Tough question starting with the fact that realistic relationships vary widely in their scope of wholesomeness and functionality; and stylized or escapist relationships can also vary widely. Just look at how popular stuff like Twilight or 50 shades got for example.
I’m going to side here with people saying it just has to be well written or entertaining enough. It’s more about the characters and the story than whether or not they get along very well or if they have struggles.