Ashton Kutcher and Demi Moore on May 12, 2011 in Los Angeles, CA. Credit: Amanda Edwards/WireImage

Ashton Kutcher is still legally married to ex Demi Moore -- but that hasn't hurt his five-month romance with Mila Kunis!

"They're really serious," an insider tells Us Weekly of Kutcher's relationship with Kunis, 29.

And seriously getting under Moore's skin. "She's not psyched about Ashton and Mila," a Moore pal admits.

Her main issue: Kutcher's longtime bond with his former That '70s Show costar. "She was always insecure over the course of her marriage," the pal says of the actress, 49. "So yeah, it's hurtful that he's with someone he knew the whole time he was with her."

Kutcher, 34, and Moore announced their separation last November in the wake of his cheating scandal with then-22-year-old Sara Leal. An insider tells Us the pair are still working out a financial arrangement before filing for divorce, and their negotiations have hit a snag.

Though the Two and a Half Men star pockets $700,000 an episode, "he's so cheap!" says the insider. "He's trying to get the best deal he can."

Still, another source says, "Ashton is eager to have everything settled."