For other girls, like 15-year-old Sarah from Tennessee, the reasons are religious.
"We're still children," she said. "And I just think it's … just like in the Bible, God said it was a gift to give to married couples. It's a gift. And we're like spoiling that gift."
The ABC News poll shows that among female virgins nationwide, 63 percent say they are waiting for the right time to have sex, and 31 percent say they are waiting specifically until they are married.
For many other 15-year-old girls, the best of intentions can sometimes be derailed. Natalie, for example, faced the hard question: What was she willing to do to get a certain boy to like her?
"I liked a guy but I knew he didn't have the same feelings for me back, but I was like desperate for him to like me," she said.
Natalie said that, later, the boy she liked told her something very hurtful.
"That the only reason he even talked to me was because there was a chance that he might get something out of me, and that I was totally not attractive, or his type, or anything," she said. "And um, and that was kind of a wake-up call."
The ABC News poll showed an astonishing 70 percent of teens said their first time had been unplanned, such as Tiffany, who was 14 when she lost her virginity.
"I kind of regret the decision to do that when I did it," she said. "Uh, honestly, I was at a party. I was drinking. I was really young, and I didn't really know what I was doing. I didn't know what I … was getting myself into. And … nobody had really told me about it, and so I was just kind of like, 'Oh, OK. Time to do it.' Like, I didn't know."
For so many girls, "the first time" is not filled with the movie magic of moonlight and music, but instead with regret. Our ABC poll shows that about 51 percent of girls wish they had waited longer to have sex.
How girls feel about themselves plays a huge role in their decision to have sex. Girls with low self-esteem are three times more likely to do something sexual that they didn't want to do, according to our ABC News poll.
While some boys are busy pushing girls to have sex, do they really like the ones who do? "Primetime" talked to some boys about it and brought the tape back to show the girls.
"And if the girl gets with all these guys. … If she had sex with everyone, [she's] like a bike that gets a ride," said one boy Johnny, with a laugh.
"No one wants to get with a girl like that," said another boy.
And no girl wants to get called the name that goes along with it -- a slut.
While boys who have sex with different girls are called "players," it's a very different world for the girls who do it. They are, as one girl said, "ruined."
"The reputation for that girl. … Like her name would basically be trash," Betty said.
Girls will even use the threat of being called a slut against each other.
Tiffany said that she hadn't done anything with a boy she was friends with, but because an older girl liked that boy too, she and her friends began to "terrorize" Tiffany.
"They would drive to my house, and they would be out in like their car, beeping and screaming, 'Whore, slut!'" she said. "They would drive by me in the park and do it to me, in the middle of the day, when I was with my friends. Like it got to a ridiculous point, so I left my school."