Mehhh..... the government shouldn't be picking winners and losers and influencing whether people are single or not. People should be able to remain single without all of the disadvantages imposed on them by the government.
Part of government's task is to help steer our society towards prosperity and stability.
It has a number of levers it can pull. Prison and fines are one lever. Tax breaks and Tax credits are another.
It is in the overall, general best interest of society that we have sufficient births to provide an adequate number of people in our society. It is in the overall, general best interest of society that those children are raised in a stable environment -- that would be a dependable, predictable one in which the child is likely to grow into a capable and responsible adult -- as opposed to an actual stable with sheep, goats and horses. (This being Christmas Eve as I write this... :) )
Government actions that help meet those needs will tend to be better for society long term than actions that hinder those needs.
That's the valid justification for picking "winners" and "losers" on various topics, marriage included.
The other one, of course, being political donations, which often contravene the public interest for some private interests.