Reasoning in my harem manga? Blasphemy.
About the girl with supposed trauma, my opinion is, like any teen in her formative years they are figuring out many things she had the boys attention even if she herself didn't wake he interest to boys yet ( if she is gay i can't say it ), the thing with the memory is that she knew boys had an interest in her, she "dumped" one and girls from her class started talking behind her back and like any teen, any shame in that age can have strange effects ( it varies from person to person sure ).
And most of all even if you have doubts and call stupidity in her supposed trauma ( can't say i don't see why ) remember she didn't plan this little prank, she didn't know about it and clearly didn't approve, she doesn't like him as a boy but she doesn't have anything against him as a person so i think it is pretty reasonable in a way, if she was actively trying to do him wrong i would understand hate here but she seems a good girl.