THERE’s only one thing standing in the way of CHARLIZE THERON and SEAN PENN’s happily ever after – her mom!

Sources tell The ENQUIRER the South African Oscar winner has fallen head over heels for the badboy actor and is anxiously waiting for him to pop the question, but her mother, Gerda Maritz, is dead-set against the union.

“Charlize says for the first time in her life she can see herself settling down with someone,” said a friend. “She’s crazy about Sean and would say yes in a second if he asked her to marry him.”

BUT that’s troubling talk for Gerda.

She thinks Sean’s a hothead who’ll only cause heartache for her famous daughter. Gerda, who helps Charlize raise her 2-year-old adopted son, Jackson, has been trying to persuade the actress to run as fast as she can AWAY from Sean.

“Gerda is counting on the fact that Charlize has always listened to her, and she’s filling her ear plenty,” the friend revealed. “She believes a marriage would be a disaster.”

Despite being one of America’s top actors with two Best Actor Oscars under his belt, Sean has seen his share of trouble. He’s been arrested six times – most recently in 2010 for kicking a photographer.

And sources recently told The ENQUIRER that the 53-year-old star was using Charlize, 38, to get back at his ex-wife and mom to his two grown kids, Robin Wright, for hooking up with her much younger “Rampart” co-star Ben Foster.

But despite Gerda’s objections, the romance seems to be going strong. The couple spent New Year’s Eve at Sean’s Hawaiian getaway and were seen cuddling at his Help Haiti Home benefit in L.A. on Jan. 11. A few days later, they were spotted in an L.A. supermarket with Charlize’s son. While she was picking out groceries, Sean kept the tot entertained by playing with him and making funny faces. Charlize looked smitten, even patting Sean’s butt as they left the store.

“To Charlize, they already feel like a family,” the friend said. But Gerda isn’t letting up.

“Gerda believes Charlize will eventually take her advice,” the friend added. “She wants nothing to do with Sean and hopes Charlize will see the light before it’s too late.”