Bad people don't tend to go around being bad unless they think they can get away with it. Once someone has pinky swore to dedicate their life to them, no matter what they might do, the true colors come out. I've seen it too many times, the idea does more harm than good.
Life-long good marriages exists, I hope to achieve one myself, but the silly oath doesn't help them in any way, and it makes the bad ones worse.