It's true that the step-parenting situation is always the result of something off-kilter that happened in the past--be it a divorce, untimely death, abandonment, or unmarried parenthood. Even in the case of an adoption, "something off-kilter" is related to the biological parents. It's important though, to assign rights and wrongs to the correct column! Perhaps there was never a sin at all, or perhaps there was a sin somewhere in the past. But the presence of children is never wrong. On the contrary, it shows that the parents made the right choice in bringing a child into the world.
The step-parenting situation, where an increasing number of us find ourselves, is the embodiment of an important consolation for past wrongs, and an even more important hope for the future. Out of every bad situation, God offers an opportunity for joy: the kids.