According to Wikipedia, the song is about falling in love. It was written by June Carter when she was struggling with her feelings for Johnny Cash. Well, also consider the fact that they committed adultery with one another. It's certainly not a love that I would wish to emulate.
The fact that they both straightened their lives out after their sin, and then lived a committed, loving and Christian marriage for 3 decades 'till the day they died doesn't at all play a role in your opinion? It certainly doesn't make what they did right, but then if everyone's life was judged only by sins they committed 30 years ago, we'd all be in BIG trouble.
(heck if I was judged by my sins yesterday, you'd think I was the biggest jerk in the world) Sometimes people change. Sometimes they don't. But putting aside how they got together, (which certainly isn't any fairy tale or moral example) I cannot think of a "public" marriage
more worthy of emulation than Johnny and June Cash's. But I admit, I'm bias, as I'm a Johnny Cash fan.