More marriage, less poverty. Well, good.
However, there is a third possible arrow of causation here: that the type of people who marry may also be the type of people who work. Creating incentives to marry may not lead to more workers, and creating incentives to work may not lead to more marriage. I would hope that there would be at least some gain, but there's no guarantee of that.
What would be most interesting is finding that one actually is a strong driver, allowing us to make a dent in one by encouraging the other. The Scandinavians try to encourage people to have more children with incentives for childcare, maternity and paternity leave, and generous subsidy. I'm not sure that's working out as they hoped.