Commitment

My grandparents were married for over 50 years. As a child I had heard the story of how three weeks after they started dating they were married. Even as a child I marveled that they barely knew each other, yet stayed married for ever. Recently, I discovered that my grandfather, who I knew to be a Christian man, had a love child about my age. When I first heard about this I couldn't believe it. So I ask his daughter, my aunt. Sure enough it was true. She told me it was a girl about my age, as far as she knew living in the area. I ask if she had ever met her, she told me no and she didn't want to. There used to be a house trailer parked on the lot next to my grandparents home and apparently this woman lived there. Now I never heard my grandparents ever utter an angry word to one another. They went to church regularly (Salvation Army). I still have trouble wrapping my head around this. Apparently my grandfather never supported this child. I don't know how he could, they were barely getting by (but that is no excuse). I can't begin to imagine how my grandmother must have felt. I recently came in posession of her Bible. I have been all through it and there are lots of notes, pieces of paper, etc. but nothing to even indicate that she was the least bit unhappy.
They stayed married through it all. Today, people are divorcing because they just get tired of one another or any other silly reason they can think of. What happened to commitment?