Honestly, i don't think the fandom understands that people who ship minor characters with adult characters or incestuous relationships KNOW the ship is problematic. They also have an understanding that this subject is uncomfortable and that what they are making is uncomfortable to most. But where people get hung up is usually like the WHY and the baffling belief that they may even enjoy making this type of media. Everyone's why is different, but the most common one is Control.
The fun of fiction is that you can have control over the characters and what scenarios you can put them in. If you like PureLily, then it isn't fun to imagine them going through devastating quest and almost dying for their love, but they beat the odds? It's cheesy and romantic, but it's also not real, and no real people were harmed by this story. The latest Toxic Yaoi of Shadow Milk x Pure Vanilla, people KNOW how mentally abusive Shadow Milk is and how that is unhealthy it is. But thinking of twisted and dark scenarios for them to be in is just as fun for some! They are not mentally abusing someone irl and frankly, i would rather they torture a cookie than a real person.
So when people are going, "It's not real, they are just cookies." This is what i assume they mean. Ultimately, EVERYONE IS MAKING CONTENT FOR THEMSELVES! Going around harrassing or bullying someone for your ship or headcanon you don't like makes you look foolish, and your echo chamber of friends telling you "Nah, they deserve it" are also assholes. Because no one deserves to be bullied over fictional characters. The golden rule: Got nothing nice to say. Don't say it. Also, a golden fandom rule: Block, mute, move on. And if you're making the media TAG YOUR CONTENT!! Genuinely, there are things some people cannot handle seeing, so placing a tag so we can block it from our timeline helps.
Anyway, adult lecture over, go make something that makes you happy.
TLDR; People who make fucked up toxic pairs and headcanons know it's fucked up, like you they can do whatever the fuck they want with the cookies! Block, mute, move on. Its better for your mental health.