Vogue

Charlize Theron never expected to fall in love with her good friend Sean Penn.

"I really didn't see it coming," the 38-year-old Oscar-winning actress told the June issue of Vogue. "Sean and I have known each other for 18 years. We were just really good friends. And it's been slow going because you are aware that you could screw that up and lose it. We had to think about it. But at the end of the day, it was somewhat effortless. It just kind of naturally happened."

Theron's romance with the 53-year-old actor, who has also won an Oscar, comes after a four-year stretch of being single.

Sean Penn, Charlize Theron Heating Up

"I was enjoying mommyhood," Theron said, about her adopted son Jackson. "You just naturally become kind of selfish about your time, and all of it is dedicated to this little thing, whether he needs it or not. And then they get into more of a schedule and you start to think, 'Wow, wait a second. There's this passionate creature in me.' And going about dating in your late 30s is such a different ball game. I know so well what I want to fit in my life. And the simple answer to that is: Unless he can make my life better? My life is really pretty f****** good."

Then, Theron's friendship with Penn turned romantic.

"Before I knew it, I was in something that was making my life better-the people who really love me can see the effect it has had on me."

Don't expect Theron to walk down the aisle with twice-married Penn anytime soon.

Love Lives of Sean Penn, Robin Wright and their Kids

"Let's put it this way: I never had the dream of the white dress," she told Vogue. "And watching other people getting married? I think it's beautiful for them, but to be quite honest, usually I'm sitting there just devastated."

Laughing, she said, "It's supposed to be this night of celebrating love, and all you see is a couple separated all night making sure everybody else is OK. It just looks like a lot of work. And as you get older, you start sifting through the stuff that really matters."

For Theron, her son tops the list. After ending her nine-year relationship with actor Stuart Townsend, the actress, who has wanted to adopt since she was very young, brought home Jackson.

"Even in my relationship with Stuart, that was something I was very clear about," Theron told Vogue about her desire to adopt. "We had very different ideas of what a family looked like. There's no judgment; that was not the reason why we broke up, but when we did, adopting was not a last resort for me-it was almost like a first resort. And it's the only thing in my life that's surpassed how great I thought it would be."