1. Young adult literature can expose teens to sexual content. This puts it in the teens mind that sex or other sexual activities are a "must do." Similar to the argument of "do violent video games encourage violence?" does seeing sexual content in books normalize young sex?
2. These books may romanticize things that shouldn't be romanticized (ie: mental illness, death, violence, cancer, etc.)
3. YA can put unrealistic ideas into teens heads (ie: being a millionaire by age 20 only by hard work, falling in love with your soulmate after meeting them a few hours prior)
4. They might not accurately portray the life of teenagers, therefore misguiding teenagers. They are written mostly by adults and some haven't been teenagers in a long time, so they might not know what teens are struggling with now days.
Hope this helped!
~Just a girl in love with Shawn Mendes