So here’s the thing: it really does not sound like you're asking this question because you want my answer, it sounds like you want to be angry with me and have a fight. And fair enough! I'm not terribly interested in a fight, but apparently this is my day to dive into this topic as thoughtfully and honestly as I can be. Maybe I'll say something you haven't already heard from other people before. Maybe not! Only you, anonymous asker, know that.
To begin with, you got part of the gist right. Real life rape (including child abuse/child sexual abuse as well as incest) is bad. Stories about rape, about underage sex, and about incest, are stories.
They're stories. They're pixels on a screen. They're not real. Whether they claim that rape is good, or bad, or sexy, or melodramatic, or life-destroying, or a normal Tuesday afternoon. They're stories.
And having a negative reaction to them is valid. Stories can stir up powerful emotions in people. It is absolutely, 100%, fair and valid and even normal for there to be certain tropes, plot elements, events, and kinds of content that make you upset and that you never want to see in a story you read, ever. You don't have to want to read about sex. You don't have to want to read any of it. That doesn't make you bad.
There are tropes, plot elements, events, and kinds of content that upset me. There are stories I won't read. The same is true of literally everyone else I know. Even though I know the stories aren't real. Even though I know the things happening in them are happening to fictional characters, who do not exist, who I cannot protect and who also cannot be harmed because they're not real. Even then, I can be made sad and scared and upset and hurt by reading those stories. And that is okay and that is valid and I am not bad or wrong for being upset about the story I've read, and neither are you.
But that doesn't mean the story doesn't have value to somebody else. That doesn't mean the story isn't important to somebody else.
What I see most often coming from antis, possibly even including yourself, is an overwhelming desire to protect. They want to keep themselves and others--possibly people they know, possibly hypothetical people they may never meet--safe from being hurt by these stories. And that desire to protect, also, is normal. It's even admirable! The problem, though, the thing that does more harm than good, is when that desire to protect drives people to lash out against things that matter to other people.
There is a difference between actual rape and stories about rape. There is a difference between a story that could theoretically hurt somebody, someday (which is all stories, always), and a story that hurts you personally. And there is a difference between a story that hurts you personally, and a story that is inherently poisonous to everyone who touches it.
We know--absolutely, scientifically, incontrovertibly--that stories about rape do not make people rapists. Yes, even the stories where the rape is there to be sexy. Even stories where the person being raped is a child. Even then. Fiction is not the same thing as normalization; again, there are far smarter people who have written far more extensively on that topic than I, and next time I come across something that goes more into detail on this point I promise I will reblog it. If this really is the thing you're afraid of, I may not be the right person to convince you that this is an unfounded fear, but I know someone out there can elaborate on it.
(Unfounded, which is not the same thing as invalid. My mother's claustrophobia is unfounded; it flares up in many situations where there's no physical threat whatsoever, where she has plenty of space to move and air to breathe. It's still real. It still chokes her. It's still valid, she is not bad or broken to feel that way, and she still can't drive through certain tunnels. The fear is real. But the thing she's afraid of can't physically hurt her, and that is worth knowing in terms of how she deals with it.)
We know, absolutely, scientifically, and incontrovertibly, that stories about rape and many, many, many other things can hurt and even traumatize their readers. Even though the situation you're reacting to is not real and you receive no physical injury, you can still be hurt by it. The key word there, though, is readers. The fact that the horror genre is out there terrifying people who enjoy being terrified for fun does not damage me unless I do something stupid and try listening to the Magnus Archives again and end up tense and miserable and paranoid for the rest of the week. The fact that guacamole is apparently delicious to everybody else in the world does not hurt me unless I do something stupid and order the wrong thing at a restaurant, and end up itchy and miserable with a little trouble breathing for the rest of the night.
The fact that there are, yes, tens of thousands of fics on AO3 in which characters under the age of 18 have sex? It can't hurt you. Those fics do not hurt you by existing. They can only hurt you if you read them. They can only hurt anyone who reads them. That's why there is an 'Underage' tag--and it's worth noting, 'Underage' is a warning, not a category. Nobody wants you to get hurt reading the wrong fic, any more than the sushi chef wants my throat to swell up because I ordered something with avocado. Literally nobody wants that.
The flip side, of course, is that you hating each and every one of those fics individually and as a group doesn't actually hurt me, or anyone else who writes, reads, or enjoys them. By itself. You can hate anything you like, and fic writers can write anything they like, and it all comes out in the end, more or less. Except.
Except that reading fic is always, entirely, 100% opt-in, and online harassment isn't even opt-out. Some antis have a nasty habit of going after writers whose content they don't like; climbing into inboxes and comments sections, calling those writers nasty names, throwing around cruelties and aggression and insults. I know that's not the same thing as simply disliking a genre, or even passively disagreeing with its existence (although disliking a genre and disagreeing with its right to exist are also very different things). I know not all antis do that. I don't know you, anon, but based on the speed and aggressiveness of this response to my last post, I can't help but wonder if you would do that.
And that does hurt people. Just like it might hurt you if someone threw a bunch of content that makes you uncomfortable into your inbox. Including the harasser, actually--because getting into fights with strangers on the internet about things that make you angry, sad, defensive, and upset isn't good for anybody. Including both you and me.
Anyway, after yet another lengthy ramble, let's get the tl;dr response to your ask here: nobody is ever bad or wrong for disliking certain content in their stories, no matter what that content is. You and your emotions are valid. The "overreacting and making things worse" part isn't about what you feel, but what you do with it. Constantly engaging with places where the thing that upsets you will probably show up, even to argue and try to fight it, will make things worse in the sense that now you're spending way more time thinking about this thing that makes you upset and angry, thereby leaving you more upset and angry. Getting together with a bunch of your upset, angry friends to make your feelings everybody else's problem? Makes fandom a more toxic place for everyone else involved.
Don't read stuff that's going to hurt you. Don't make other people read stuff that's going to hurt them. That's the whole thing, really.