I just want to read them for fun . I’m 100% sure I will not do anything haram afterwards. Please be convincing.
All my answers are based on my knowledge about Islam being a born Arab Muslim; a knowledge gained by watching sheikhs on TVs, going to Jumaa prayers, reading books including the Quran, and listening to hadiths. I do not claim that my answers contain the absolute truth, but I'm trying to use my humble knowledge to help people, but being a human being, I am still prone to errors and mistakes.
Allah Knows Best.
These stories are written to brighten the flame of lust in their readers. Such stories don't have an actual story that one can learn from, but rather a detailed description of a sexual fantasy. The "fun" you're talking about is sexual arousal. This "fun" you're talking about, whether you act upon it or not, should only be gotten through your spouse, and only through him/her.
I will answer your question with a question:
Is it halal or haram to watch pornographic scenes if I'm 100% sure I will not act upon them?
Your question is similar but you're replacing the visual porno with your imaginary one. When using your imagination, are you imagining a halal relationship? Are you visualizing your husband or a random guy, the character in the story?
Allah says in his holy Quran:
Allah Knows of the (tricks) that deceive with the eyes, and all that the hearts of men conceal
[...] And know that Allah knows what is within yourselves, so beware of Him. And know that Allah is Forgiving and Forbearing.
One final note: except for prophets, every single human being is prone to falling for his lust. The number of young muslims who were very pious and religious, who thought they would NEVER fall for their lust, and ended up in haram relationships; that number is very high that you can't count them all, and almost all of them started with minor actions that would enhance the flame of lust in them, one action after the other. So you can't be
100% sure I will not do anything haram afterwards
Despite all that has been said up above, even if it's not haram, if it creates the slightest chance (0.01% chance) that it might lead to to something haram, avoid it for your own sake. This is closer to iman and closer to taqwa and Allah will reward you for it inshallah.