A report published earlier this week found that children in 2-parent families are at much lower risk of developing behavioural or emotional problems. The research is based on interviews with over 5,000 children over a 3 year period.
The report says: “The odds of developing an emotional disorder were increased for children where there had been a change in the number of parents between surveys, from two parents to one parent compared with children and young people in families that had two parents at both times.”
There have been several other bits of research which point this out, but the fact that this is a Government report may give it a bit more weight. At the moment, the Conservatives are the only political party which has policies to recognise and support marriage, whilst Labours Harriet Harman is on record as saying that "there is no 'ideal' parenting scenario" and "marriage has little relevance to public policy".
Other sites following this seem to just be quoting the Telegraph report, and the original government report doesn't seem to be online (unless I was using the wrong search terms), so the link above is all we have to go on for now. A link between losing 1 parent and emotional problems is what you'd expect, but it would be interesting to see if the children who had 1 parent at both points in the survey (2004 and 2007) showed any difference from those who had 2.