Marriage is not a guarantee. But it does put social, legal, and pain-in-the-butt roadblocks to parents breaking up. Particularly if we repealed no-fault divorce laws. Its hard to argue that these laws helped anything.
The best way to encurage two-parent households is is through social stigma to pre-marital sex and cohabitation.
If you're familiar with Defoe's Atlantic Monthly article (which I just scanned again), then you'll remember that with few exceptions, there are no desirable results for children in single-parent households.
If the best home for children is one with two parents, then why should they not marry? If only to give the child a name shared by every member of the family, I think its a very good idea.
Because there is no real concrete value in doing so? Particularly when one considers the disadvantages. It is the reason a lot of people my age-ish can be in a very long-term committed monogamous relationships without a marriage certificate, even if they have kids. "Marriage" as a state institution adds precious little to life. Maybe it is a generational thing, but I and many others I know do not buy into the idea that you need the permission of the state to do anything related to what you consider "family". If I and someone else wants to build a loving and functional family, to hell with the state just out of principle. Freedom of association and all that. The rubber stamp of the state is pure detriment and unnecessary.
Eliminating no-fault divorce is a sure way to guarantee that even fewer people get married. People advocating that are trying to ensure the quick death of marriage, and any contract that can't simply be dissolved by mutual agreement of the two principals is a dysfunctional construct anyway.