I've seen this in only two situations where I have personally known the people involved.
1. Girl got knocked up, he married her, she cheated with his best friend and got knocked up again, they divorced.
2. Girl decided it was high time to get married and "oops, I'm pregnant!" They married, and then she miscarried
They are still together and have two children.
To be honest, most of my friends who have become pregnant outside of wedlock and decided to keep the baby have either had the baby with their current partner and remained unmarried (at the time - some got married LATER but not "as a direct result" of her pregnancy) or have gone the single mom route.