The celebrity couple -- who both starred with Danny on 'That '70s Show' -- have been getting crucified online after it came out they each wrote letters to the judge in DM's sentencing hearing ... in which they said he was of good character, and deserved mercy.

Of course, a lot of people didn't take too kindly to that ... and it seems Ashton and Mila are fully aware of that, 'cause they just posted an apology video explaining their intentions.

Check out how they break this down for yourself -- MK and AK say their letters weren't meant to discredit the verdict reached against Danny, nor was it intended to push back against what his two accusers in this case had alleged against him ... namely, rape.

They say all they were trying to do was lay out who they personally knew Danny as, and what their experiences were like with him -- this after being asked by his family to do so.

Mila and Ashton say they advocate on behalf of victims of sexual abuse, and apologize to anyone who was hurt by their actions. This last point is interesting, Ashton specifically was getting called a hypocrite -- on account of his work combatting human trafficking.

His testimony to Congress a few years ago -- where he passionately laid out his mission to protect victims of abuse -- has been making the rounds anew ... and drawing side-eyes.

It'll be interesting to see how the public takes this mea culpa. Fact is, a lot of the dynamics of the show back then are under a microscope right now in the wake of Masterson's imprisonment -- including a look back at how Topher Grace was portrayed at the time.

BTW, it's worth noting ... Kurtwood Smith and Debra Jo Rupp -- who played Red and Kitty Forman, respectively -- also wrote letters to the judge on behalf of Danny ... but haven't received nearly as much backlash as Ashton and Mila did over the past 24 hours.

The other prominent cast members -- including Laura Prepon and Wilmer Valderrama -- have remained silent.