Creatives aren't supposed to be creative?
Harems aren't creative. Incest isn't creative. They are structures or tools within which you can create. Think of them as different paintbrushes used on the canvas. The author/artist is the one being creative in how they use the tools, and what they come up with in the end.
To that end, there's nothing inherently wrong with offering either Harems, or Incest in an AVN, provided they aren't pushed onto the player with no opt-out. If you don't want a harem, don't make one. If you don't want to sleep with your sister/mother, don't. That is of course, the author was good enough to make them optional, otherwise you have to choose whether or not to even play the game.
But again, I don't see them as negatives - just as tools to use to expand a story's possibilities.