The boys were all at my house for a few hours yesterday for Fathers Day. Mid-way through the visit, Youngest Son looked up and me and said "Daddy, I thought you and Mom were going to stay married forever."
I was more than a little surprised. I told him "I thought so too. But it turned out we didn't want to live the same way." He asked my why. I told him we just did. He asked again. I told him we just did, and that's all I can tell him right now.
Oldest has asked me many times. I eventually told him that I hadn't wanted a divorce, that it was his mom who decided to do it. Someday I will probably have the discussion with Youngest as well. But not today.