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 Particularly if we repealed no-fault divorce laws.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.
I hope your real question is how do we get more heterosexual two-parent households who stay together. Homosexual "couples" are not parents just because they take in children as pets or spawn toys (and in all likelihood become objects of pedophilia by themselves or their lovers.)
Here's the answer:
(1)Reduce the legal incentives for divorce. Loose divorce law is what has been eroding marriage steadilly for the past 50 years (lawyers will hate that but who cares?).
(2)Instead of imposing a marriage penalty in the tax code, not only remove it, but lower tax rates for couples who stay married longer.
Homosexuals should never have rights of parentage and as a matter of fact should be classified minimally as mentally ill, and simply unfit, period. Theirs are twisted minds with a twisted agenda born of self hate, and their purpose is the destruction of that which is good and wholesome, starting with sexual relationships and taking it straight through to the destruction of the institution of marriage.