How often is it that couples REALLY fall out of love? Once in a blue moon… that or they find someone else to do the nasty with.

While Robert “Mutt” Lange says that the reason for his breakup with longtime wife Shania Twain was “literally just growing apart,” PEOPLE sources say the recluse producer was having an affair with the couple’s secretary.

According to the PEOPLE sources, Lange was romantically involved with Marie Ann Thiébaud, who was a secretary and manager of Twain and Lange’s Switzerland home. And despite Lange’s denials, one source says that he and Thiébaud “left their spouses for each other and are still in a relationship.”

Lange also denies having any affair. “It’s absolutely not the reason [for the separation],” the he told PEOPLE.

A PEOPLE source reports that country superstar Twain is “devastated” over the split, which “came out of left field.”

Twain, 42, and Lange, 59, have been married for 14 years. They tied the knot in 1993 and have a 6-year-old son, Eja D’Angelo, together. (Source)