"Someone at school told me about it," Dunham, now an ambassador for Planned Parenthood, tells PEOPLE of learning all about the birds and the bees at 7 years old – and wanting her mom, artist Laurie Simmons, to tell her that what she heard wasn't true.
"I said 'Oh my God, Mom, this girl, Amanda, told me this insane thing that sex is when a man puts his penis in a woman's vagina,' " recalls Dunham, 28. "I had expected her to say that was absurd, but I saw from the look on her face that it might actually be true and I fell into a deep depression."
Up until then, Dunham had another idea altogether. "I was convinced I had figured it out because I thought if a man and woman put their arms together, the sperm and egg met through the pores of their skin," she says. "That didn't seem too scary for me."
Once she knew the facts, she says, "I stuck it in the back of my head. I did not go back to school and tell all the other kids."
Dunham was raised in a liberal Manhattan household, where her family spoke openly about sex. "There was no stigma around me asking questions in that department," she says. "We definitely had the kind of house where you said 'vagina.' There was no pet name for vagina. That's what it's called. Which I appreciate."
As an adult, Dunham has carried on that tradition of openness – both on her show and in her work with Planned Parenthood – and makes it a point to talk about women's reproductive rights and sexual health.
In this week's episode, airing Sunday on HBO, one of the Girls characters will deal with the issue of abortion.
"A huge part of our show is an attempt to shift the stigma around women talking about their sexuality and women talking about their experiences," Dunham says. "We want women to know their stories have just as much power and are just as necessary as anyone else's."
For more on Lena Dunham and her support for Planned Parenthood, pick up the new issue of PEOPLE, on stands Friday
Lena Dunham speaks during the Planned Parenthood Action Fund's Sex, Politics and Film event
Sonia Recchia / Getty
