A fifteen year old may enjoy the attention and not realize that she's being used. Few moms explain it to their daughters in extremely clear terms that they can understand, such as pointing to a home appliance like a toaster or blender and saying, "That's you."
The adult women in their lives stick to politically correct topics such as compatibility and condoms, and possibly marriage. Celebrity pop stars in the media dress like prostitutes on drugs and they have a new boyfriend every few months. Teenage girls don't have a brutally honest mentor to teach them how to recognize when they're being used.
If they're lucky they'll read a fashion magazine that mentions it. Magazines do but it's written for an adult audience. Usually the focus is on whether one's boyfriend gives one pleasure: If he doesn't than he's selfish and the relationship is not real. Men like that view women as sex objects, like they're home appliances, but lower because they're also disposable like Kleenex tissue. Obviously teenage girls are not supposed to read that but those who do become informed about the ice cold reality that having sex no longer implies that they're in a relationship.
A fifteen year old who hasn't learned that yet may think that it's cool to date an adult but everyone else knows that he's going to leave her, which is the reason why statutory rape is such a big deal.
The reason why incest is labeled as the Ultimate Betrayal is because it's exploitation by someone who is known and trusted. It's also humiliating for the victim because s/he knows that it's forbidden and if anyone finds out then the consequences will ruin both of their lives. For example, Child Protective Services ("CPS") would remove the child and put him or her in foster care, and that may be worse, and so victims have a strong incentive to remain silent.