Charlize Theron and Sean Penn reportedly butted heads over marriage. Earlier this month news broke that the pair had allegedly split after nearly 18 months together. It was thought they'd become engaged late last year, but now sources say it was the subject of marriage which was one of the reasons behind their separation. "Charlize fell for Sean hard and it was a very passionate relationship from the start. But in recent months they've been arguing and Charlize realised it just wasn't going to work. She made up her mind – and once she's done that, there's no going back. They have been butting heads about marriage for some time. She said they didn't need paperwork to prove their love, but Sean was really pushing for a ceremony," a source told British magazine Grazia. "At the back of her mind Charlize knew she could never be persuaded about marriage. They've been bickering and she couldn't help but make the decision to break up." As well as the marriage issue, the source reveals tensions arose with Sean's kids. Sean has daughter Dylan, 24, and son Hopper, 21, with ex-wife Robin Wright, and apparently they found it hard to accept their dad had moved on. "Charlize never tried to replace Robin, but it was difficult for Sean's kids to adapt to their dad having a new partner. They're adults and apparently clashed a bit with Charlize," the insider explained. Last Sunday Sean was spotted out with Dylan, celebrating Father's Day in Los Angeles.