The line, from my perspective is twofold. First and foremost, has the 0.1 or demo launched? If yes, then input should now and forever forward be technically based. If there's an issue with the writing, the characterization, the art, the gameplay, etc, these things should be addressed and fixed as needed. However, if the game has launched and you have a problem with the content, be that NTR, or Futa, or Incest, you're barking up the wrong tree. The developer isn't going to, and shouldn't change to appeal to you.
However, if a developer is doing something like what
@Kane 'Vincidon' Salvatore and
@Xander 'Clyde' Salvatore are doing with their thread asking for input and ideas to flesh out their own vision before the game's launch, including and incorporating people's ideas into their story, then and only then, is it time for you to bring up your personal fetish and see if it can be added to or taken out of the running for how this game will advance. Their game, as you'll find if you read the thread, is moving along into the first release now, they have a pretty solid framework, and it includes 2 of the fetishes that are the most controversial. The theme is father daughter incest, but the daughters, all 3 of them, aren't going to be doe eyed girls obsessed with their daddy, they'll have their own loves and lusts beyond him, just as he will have beyond each of them individually. NTR haters will freak out over this, but it's just the vision they have for how something like this would play out.
Anyone going into their thread, at this point, needs to acknowledge, that much of that has already been decided. Input on other aspects, character models, clothing and environment assets, etc, is still appreciated though. And that leads me to how developers can and should ask for input. Polls, Summertime Saga, Dreams of Desire, and my team's project Game Legends, are all using polls to determine certain directions we'll take. DoD uses them to add flavour with alternate outfits for higher tiered patrons. SS uses them to decide which line they'll flesh out next. GL has us using them to determine what characters people would like to see us parody. The story is still up to the dev, the content is still up to the dev, but the public gets to influence things that they'll be satisfied by.