Miranda Lambert says she knew husband Blake Shelton was married to another woman when she met him at CMT's 100 Greatest Duets Concert.
The two sang the duet "You're the Reason God Made Oklahoma," a 1981 single originally performed by David Frizzell and Shelly West, on stage at the gig, which aired on the channel in 2005.
"I knew he was married," Lambert, 27, told NBC's news show "Dateline" in a recent interview, which aired on Monday.
"I had seen their wedding pictures in Country Weekly." I knew better, like, this is off-limits. My parents are private investigators, for God's sake. I've seen this my whole life - affairs, and of all people to know better, I know better than this."
At the time, Shelton was married to Kaynette Williams. Their marriage lasted from 2003 to 2006. They had no children. Lambert and Shelton began dating in 2006 and he proposed to Lambert in May 2010. The country couple wed in May 2011. It was the first marriage for Lambert.
The singer walked down the aisle wearing her mother's wedding dress. Guests included Reba McEntire, Martina McBride, Cee Lo Green, Katherine Heigl and her husband Josh Kelley, and Kelly Clarkson.
Lambert is known for her hits song like "The House That Built Me" off her 2009 album "Revolution." Shelton also has a successful country music career with his hits "Hillbilly Bone" and "Austin." Shelton is also a coach on NBC's "The Voice."
She also said on "Dateline" that she is "very insecure" about her physical appearance.
"I'm just like every other girl. I have to get on stage and wonder if something jiggling or if I've gained an extra 10 pounds and I'm on TV. It's a constant issue. I have so many girls that come up to me and say, 'Thanks for being normal.'"
"Because I'm a normal-sized girl," she said. "I'm a 6 to an 8, give or take, and I'm fine with that and I've been very open about just being healthy and happy. Sometimes when I'm happy, I have a few extra happy pounds, like we all do. I love Cheetos, like snacks. That's bad. And fried food. I try to stay away from them. I try to skip the fried food and go for the salad."
Lambert talked to the Associated Press about getting in shape for wedding at the 2010 Country Music Association Awards.
"I've sort of been gradually working down, losing some weight and working out," she said at the time. "I definitely want to be a fit bride. So I'm just trying to keep myself in a good routine, a healthy routine."
The country music star is gearing up for the debut of her fourth studio album "Four the Record," set to be release on November 1.