Britney Believed Justin Was the One

N E W   Y O R K, Nov. 13 -- Britney Spears may look all-grown-up these days — but her youthful views about the lasting power of a young first love set her up for a difficult year that took her through heartbreak, acceptance and discovery.

The young pop star who was once famous for her virgin status says she now believes that waiting until marriage could be an unrealistic goal for longtime couples, but casual sex is still a no no.

"I think in reality, if you really know in your heart of heart that you are gonna be with that person, forever, and you really think that, with all of your heart, and you're in four, or five years, you know, into the relationship, I mean, stuff can happen," Spears told ABCNEWS' Diane Sawyer in an exclusive Primetime interview.

'You Don't Go Around Sleeping Around'

Justin Timberlake — Spears' longtime boyfriend from back in their Mouseketeers days — revealed his ex-girlfriend's sex status in an interview on ABCNEWS' 20/20 earlier this year.

Spears, 21, says she believed she and Timberlake had a relationship that would last a lifetime. She and the 22-year-old 'N Sync singer were together for three years before a high profile break up in the spring of 2002.

"You know, I mean, I, you don't go around sleeping around with people," Spears said. "I mean, I really thought that I was gonna be with him or the rest of my life. I did."

Spears has said she's not currently involved in any serious romance, but still leaves room for school girl crushes.

In fact, she's a little embarrassed about one.

When Sawyer asked about a framed picture of Spears and Brad Pitt, who is very taken by Friends star Jennifer Aniston, the mega star cringes.

"We shouldn't share that. I'm embarrassed," Spears said. "That's my little thing."

The First Time

Spears took command of the pop music world when she was just a teenager. Her first CD Baby One More Time (1999) sold a staggering 10 million copies. Since then, she's left girlhood innocence far behind, reinventing herself as a steamy sex symbol.