And things have gotten serious – fast. "It's early, but they have talked about marriage," says an insider.
While home in Los Angeles, the duo have been nearly inseparable. "Benji spends so much time at Cameron's that it seems he now kind of lives there," says another source. "They look very happy together … Cameron hasn't looked this happy for years."
Last week, the pair took their romance to a new level on a romantic European getaway, sailing along the French and Italian coasts aboard the $633,000-per-week, three-story Lady Britt mega yacht, with friends in tow.
Madden, 35, is going the extra mile, having met Diaz's family on a recent trip to Florida.
"It seemed important to Cameron that Benji met her family, and she looked very proud to introduce him to everyone," says a witness. "They were very sweet to each other, constantly kissing and flirting. It was obvious that they had a great vacation."
Reporting by PERNILLA CEDENHEIM and LINDA MARX
For more on their romance, pick up the new issue of PEOPLE, on newsstands Friday