"This was an essay I was very anxious and self-conscious about putting in the book because we are in a current culture where everything is turned into a game of telephone and it turns into a headline," she said to Howard Stern today on his radio show.
"[But] we are living in a moment where campus assault is an epidemic and the amount of young women who don't feel safe in their own college campuses or violated by people they know, then blame themselves because they are indulging in typical college behavior ... and so, I think so many of these young women are speaking out."
The 2008 Oberlin graduate continued, "It gave me the strength to talk about something that I'd been through that had been very challenging to make peace with."
The "Girls" star and creator, 28, said she hadn't talked about the assault publicly yet because she thought it was such a "huge, but private part of her identity." She also didn't want to be labeled a victim.
"I've long known how empowering it is to talk about your experiences, but this was one I was very afraid to touch," she added. "But also, I went to a small college, people knew each other."
Dunham continued about her experience that she thinks "so many people don't understand what the meaning of consent is," regarding sex.
She said kissing doesn't mean the girl has agreed to do everything.
"If you are in the middle of having sex and someone suddenly becomes forceful with you and makes you do things you don't want to do, that is the act of sexual assault," she said. "So many women go, 'But I put myself in this situation.' ... that is a very dangerous mythology that we have to break down."
Indeed, Dunham said she didn't go to the authorities and didn't fully tell her parents what happened.
"I was physically hurt, so my mom made an appointment with to go to the doctor, but ... I had told [my mom] that I had gotten drunk and gotten into a rough situation," she added. "I said, 'Oh, I don't really know if we used a condom, I feel like we should go to the doctor.'"
Finally, Dunham "implied" to the doctor what had happened, who was sympathetic.
"My best friend Audrey knew, she was the only person who knew," she said. "I think there was a big part of me that thought people would say, 'You're lying, you exaggerated. He's more attractive than you are. Why would he want to do that to you?'"
While writing the book, Dunham shared the essay with her mom Laurie Simmons.
What's missing from the sexual assault conversation is that every guy that crosses the line isn't a "straight villain," Dunham said.
"Women need to learn to speak out and guys need to talk to each other about consent," she said. "There is no 'No' that means 'Yes.'"