There is economic evidence that taxes and benefits do have an effect on marriage and divorce rates, particularly among low-income households. That implies that the tax benefit system does matter when it comes to encouraging sustaining or forming marriages. But whether that means that using financial incentives to encourage marriage is a good policy, is a different question.
There are three reasons why it might not be as good an idea as it first sounds. First the effects of tax on marriage are small, so you need to spend a lot of money on the vast majority of couples/individuals who won't change their behaviour in order to encourage just a small few to get, or stay, married. Second, while people with children being in a stable loving marriage is obviously desirable, it's not clear that those unions incentivised by financial considerations are necessarily made for the right reasons. Finally, since working lone parents are already at a big disadvantage for earning and caring, compared to families with two pairs of hands, many will think that giving them a bigger tax bill to boot is unfair.
