“Boy Meets World” alum William Daniels and wife Bonnie Bartlett have been spouses for more than 70 years, but Bartlett is opening up about how their early experiments with an open marriage proved to be “painful.”

As Bartlett details in her new memoir, Middle of the Rainbow, the two “never felt tied to fidelity” in the early days of their marriage until Daniels had an affair that led them to question their stance.

“I guess it was a little bit of an open marriage at first, but that was very painful. That didn’t work well,” Bartlett, 93, told Fox News.

READ MORE: William Daniels On Why He Turned Down His Iconic ‘Boy Meets World’ Mr. Feeny Role Twice (Exclusive)

“And it was a time when people were doing that,” she continued. “It was at a time in New York when there was a lot of sex and a lot of people doing all kinds of things, you know — very free.”

She recalled “an affair that lasted a few months” that she had with an actor who was “slightly boring” in the late 1950s.

“I never felt guilty because I never felt tied to fidelity, and neither did Bill,” she explained.

READ MORE: ‘Boy Meets World’ Star William Daniels On Secret To His 70 Year Marriage

However, she adds, when her husband, now 95, had an affair in the 1970s, she was “devastated,” and realized she “could no longer tolerate any kind of open marriage.”

“It was very painful for the both of us,” she said. “But it was something we had to go through because we never went through it.”

She added, “When we got together I was 18. Bill was my first boyfriend … We just had to go through all that and still, we loved each other very much and always have… [We] have always been there for each other. That’s what matters — if you’re there for the person and help [them] along in a relationship, [have] respect for them and what they’re doing and being there for them… [You have to] be together on the other side.”