Oliver Hudson -- an actor himself -- made the revelation this week on his and his sister's podcast 'Sibling Revelry' ... explaining that not long before he married Erinn Bartlett in 2006, he was being unfaithful.

He explains ... "When I got engaged, something happened psychologically, and I spiraled, and I was unfaithful, and I was cheating, and I was crazy. I never got caught. I told her everything because I couldn't live with myself and get married and be married and have children with this sort of weight."

Oliver goes on to say he had to power through that -- and says his mom, Goldie Hawn, played a big part in helping him heal ... adding he believes he's a good man at heart.

There's also this ... OH says he and Erinn did a lot of therapy, and in the end -- he says he doesn't regret his hurtful action ... because he thinks he's better now, and wouldn't be who he is today without having gone through that.

Oliver says, "I'm not sure I have many regrets because if I didn't, it was a choice that I made, whether it was a positive one or a negative one. I've made some horrible, horrible choices in my life just generally. You would think I regret doing that, but I don't know who I would be if I didn't make that choice."

Erinn and Oliver stayed together and went on to have three children together.