Several insiders claim that Gwyneth, 41, and Chris, 37, had an "open" relationship "at times."
"They were physically separated and emotionally," says a source. "They always have been comfortable doing things apart. No one thought that was unusual."
Adds a Paltrow pal, "They have been on and off for many years. The marriage was falling apart. It caused both of them tremendous pain, and they tried to fix it day after day."
Over the course of their 10-year union, each has been put through the rumor mill, with Martin linked to Kate Bosworth and Alexa Chung, while Paltrow adamantly denied talk of a romance with entertainment lawyer Kevin Yorn, among others.
But an insider scoffs to the mag that the two-timing talk is "ridiculous."
"If Chris was playing around, Gwyneth didn't care," alleges a spy. "She's never been jealous or territorial. Her philosophy is, 'We're both adults. We make our own choices.'"
Also said to have contributed to the marital meltdown: differences in lifestyle, including his tour schedule and her movie career, as well as her anxieties about aging (supposedly leading to "flirting" and "narcissism") and their differing attitudes toward fame (she enjoyed the spotlight; he didn't).
Chris "is the kind of person who is always in a T-shirt," chimes in a friend. "She's much more glam."
The couple, who headed to the Bahamas with kids Apple, 9, and Moses, nearly 8, as the split news broke, "seemed to have a pretty mature handle on that crazy rotation of being superstars and parents," opines another source. "But that balance, given the time they had to spend apart, clearly couldn't sustain itself."
Meanwhile, Us also has a few theories on what finally did them in, and it seems all those jokes about the Coldplay frontman having a post-split McDonald's binge may not have been far off the mark …