As much as a moral compass is taught to children by parents, it is also given to kids from outside influences. When violence, drugs and sex are steeped in our popular music and our children are around it everywhere, it becomes normal to them. They see that world as something that happens in every day life, which isn't the case.
One way that modern rap and pop music negatively influences children and young adults is that it teaches them to be disrespectful to their parents. For example, kids learn to refer to their fathers as my old man rather than as their dad or father. Furthermore, many modern songs are sexually charged, and parents may not want their kids exposed to such adult themes until they are older.
Modern music contains a lot of elements that some children and teenagers might not understand as fiction. Many children and teens idolize pop and rap artists, which can alter their perception of what is real and what is not. They may not understand that a rapper writes about guns and women because it's what is popular, and the child may think that it's morally okay to behave the way a song suggests.
If I rapped about how doing good in school and eating vegetables are good for you it's still rap. The media matters very little. The ideas conveyed are what you should attack, music today is yes very thematically outputting but discriminating against a style is like discriminating against skin color.
Music, regardless of the genre, does not negatively influence teens. In fact, many teens use music to unwind and let go of frustration. In this way, music is a healthy diversion. Music only becomes a negative influence when parents fail to provide proper guidance in the lives of their teens, and the teens are forced to look for it elsewhere. Some may find it in music, but that is the exception rather than rule.