Arquette revealed today on Howard Stern's Sirius radio show why he and Cox decided to separate after 11 years of marriage.
"We're not having sex, and I completely understand," Arquette, 39, said. "She's in a place of wanting to be real and emotional. She's an emotional being. She's an amazing woman. If it doesn't feel right, she doesn't feel like bonding in that way."
Arquette added that Cox, 46, wants him to grow up.
"She says that to me: 'I don't want to be your mother anymore,'" he said. "I appreciated that. I respected that. I've been going to therapy. I'm trying to grow up."
The couple announced they were splitting on Monday. In a statement, they said they are trying a "trial separation" and that their split "dates back for some time."
"The reason for this separation is to better understand ourselves and the qualities we need in a partner and for our marriage," the statement added. "We remain best friends and responsible parents to our daughter and we still love each other deeply."
Cox and Arquette met in 1996 on the set of the first "Scream" movie. They have one daughter together, Coco, 6.
While Cox's career seemed to generate more steam than Arquette's -- after gaining fame on "Friends" in the '90s, she went on to appear in a number of movies and headline TV series, including FX's "Dirt" and ABC's "Cougar Town" -- they reunited on set over the summer to film "Scream 4," and things appeared to be fine.
"It's nostalgic," Arquette told People magazine in June about filming "Scream 4" with Arquette. "We met on 'Scream 1,' and on 'Scream 2' we were kind of on-and-off dating. And by 'Scream 3' we were married. And now on 'Scream 4' we have a child. It's sort of an amazing marker throughout our lives."
As recently as last week, Arquette seemed smitten with Cox as well. He wrote on his Twitter account from Cox's "Cougar Town" dressing room Friday.
"Courteney's 'Cougar Town' dressing room. The girl's got style," Arquette wrote, adding a photograph of the room.
And, in June, Arquette dismissed reports that his marriage was on the rocks as he and Cox celebrated their 11th wedding anniversary.
"You know, it's weird, because it turns up in the tabloids, and your friends start calling you like there's a problem," he told People magazine. "There's nothing [wrong]. We have a really great, beautiful relationship."
But Cox and Arquette's marriage wasn't without problems. Her struggle to get pregnant led her to advocate having children soon after marriage.
"In retrospect, I would say: don't wait as long as I did, especially if you want more than one child," Cox told People magazine in 2008, adding that she faced "a lot of in vitro fertilization," a process she recalled as "tough."
In the same interview, Cox also revealed that she wanted another child.
"I'd like another child, and maybe if it doesn't work out, I'll adopt," she said.
Cox and Arquette also sought help for their marital problems. In a candid profile featured in the August issue of InStyle magazine, Cox -- who didn't wear her wedding ring during the interview -- went into detail about the couple's fights.
"We've done couples therapy in the past. We're not lazy about our marriage," she said. "We have the same arguments we've had for years."