Charlize Theron opens up about her relationship with Sean Penn.

Charlize Theron & Sean Penn

Charlize Theron has opened up about her relationship with Sean Penn.

The 38-year-old actress, who stars in new comedy A Million Ways To Die In The West, and the 53-year-old Penn are rumoured to have begun dating last year and publicly confirmed their relationship during awards season when they attended various events as a couple.

Charlize appears on the cover of the June issue of Vogue, and spoke to the magazine for the first time about her new love. She revealed: "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, and 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."

The Oscar-winning actress, who adopted son Jackson in 2012, admitted it wasn't easy having such a high-profile relationship.

She said: "I don't know how you have a healthy relationship with the whole world knowing about it. Look, we're dating, and I don't think we were trying to hide anything. We were photographed; the relationship was kind of written about. I recently did a day-long press junket, which I hadn't done in forever, and it's a harsh reminder that whatever people try to hide a press junket behind, it's really a roundabout way of getting at as much private s*** as they can. It was all about what I like to eat and avoiding questions about Sean or my son."

Charlize had been single for several years, after splitting from Irish actor Stuart Townsend in 2010 after nine years together.

She said: "I was single for a really long time. I was enjoying mommyhood. 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***ing good."

And Charlize revealed that while she had always planned to adopt, becoming a mother has exceeded her expectations.

She said: "Maybe it was just an awareness of the fact that at that time, orphanages were everywhere in South Africa. But even in my relationship with Stuart, that was something I was very clear about. 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."