On Sex, Online Harassment, & Personal Privilege

A friend shared a story of sexual harassment with me today. A hapless girl’s pictures and contact information was shared within a group encouraging sexually inappropriate behavior and remarks. It didn’t matter that she was fully clothed, or that she was a human being deserving of respect. People in the group (and like-minded individuals) wasted no time in objectifying her, harassing her over private messages and the like.
Why is this so?