Ashton is leaving Thorn, the org he co-founded in 2012, and so is his wife Mila Kunis, who was an observer on the organization's board of directors.

They're departing due to public outrage over the letters they wrote to the judge who sentenced Masterson, their fellow "That '70s Show Star" who was convicted of raping 2 women. The letters were written to inform the judge about Danny's character -- in their eyes -- to help the judge decide on a sentence, but it blew up in their faces.

Ashton's resignation letter to Thorn's board, obtained by TIME, explains ... "Victims of sexual abuse have been historically silenced and the character statement I submitted is yet another painful instance of questioning victims who are brave enough to share their experiences."

He continues ... "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 as Chairman of the Board, effectively immediately."

Ashton adds,  "I cannot allow my error in judgment to distract from our efforts and the children we serve."

He also apologized for letting down sexual assault victims and other advocates, writing ... "The mission must always be the priority and I want to offer my heartfelt apology to all victims of sexual violence and everyone at Thorn who I hurt by what I did. And to the broader advocacy community, I am deeply sorry."

AK testified before Congress a few years back, passionately laying out his mission to protect abuse victims ... and video of his impassioned speech is now being scrutinized in the wake of his Masterson defense.