They assume that the sex was consensual, and that therefore the girl must be a slut. When you are publicly humiliated, you develop coping mechanisms. Often these coping mechanisms are unhealthy. Many girls turn to alcohol or drugs. Many develop eating problems, especially if their reputations were stirred up by their physique.
Many girls become severely depressed to the point of contemplating and attempting suicide.
Many girls develop a real and serious hang-up about sex that persists into their adult years. If you are a parent, teach your daughters and sons that girls as well as boys have sexual feelings, and that sexual feelings are entirely normal. That way they won't have to pin their sexual anxieties on a scapegoat and then distance themselves from her. If you are an educator, know that slut bashing, if it is persistent and severe, is a form of sexual harassment, and that it is your legal obligation to ensure that the behavior stops. And if you are a teen, don't join in with your friends if they pick on a girl as a slut.