Los Angeles, California – Actor Ashton Kutcher stepped down Friday from his anti-sex-trafficking organization following outrage over letters he and wife Mila Kunis wrote asking a judge for leniency in sentencing convicted rapist and former That '70s Show co-star Danny Masterson.

Kutcher announced he was resigning effective immediately from his role as board chairman at Thorn, an organization whose goal is to address the role the internet plays in child sex trafficking and abuse. Kutcher founded the group with his then-wife, Demi Moore, in 2009.

"After my wife and I spent several days of listening, personal reflection, learning, and conversations with survivors and the employees and leadership at Thorn, I have determined the responsible thing for me to do is resign ... I cannot allow my error in judgment to distract from our efforts and the children we serve," Kutcher wrote in a letter shared by Thorn.

In it, he apologized to survivors of sexual assault for the letter he wrote that defended Masterson's character. Neither Kunis nor Kutcher argued that Masterson was not guilty, but they vouched for the person they knew him to be.

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"The character statement I submitted is yet another painful instance of questioning victims who are brave enough to share their experiences. This is precisely what we have all worked to reverse over the last decade," Kutcher wrote.

Masterson (47) was sentenced last week in Los Angeles County to 30 years to life in prison after being convicted of two counts of rape. The May verdict followed his second trial that stemmed from accusations made by three women he met through the Church of Scientology.

"I do not believe he is an ongoing harm to society and having his daughter raised without a present father would [be] a tertiary injustice in and of itself," Kutcher wrote in his letter to Superior Court Judge Charlaine Olmedo.

Kunis, who was also a star on That '70s Show, wrote another letter on Masterson's behalf.

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"From the very beginning, I could sense his innate goodness and genuine nature," Kunis wrote. "His caring nature and ability to offer guidance have been instrumental in my growth both personally and professionally."

In a video posted to Instagram, the couple clarified why they wrote the letters a day after they were publicized.

"They were written for the judge to read and not to undermine the testimony of the victims or retraumatize them in any way," Kutcher said. "We would never want to do that. And we're sorry if that has taken place."

Kunis is also stepping down from her role as an observer on the Thorn board, according to a story in Time, which first reported the resignations.

"We would not be the Thorn that we are today without Ashton's contributions. He has played a significant role in the impact we have made, and we are grateful for his participation over the last 15 years," the organization said in a statement.