Trainspotting I think you are trying to wish some unpleasant but very real issues away.
Of course if you adjusted for every single financial and social variable in a study of single parent outcomes (if it were possible to do that) you would probably find that the disadvantages associated with having a single parent might reduce.
But the entire point is that you can't separate one from the other because being a single parent in the first place is one of the primary causes of financial difficulty! They are directly casually linked. Single parents are far more likely to have a lower socio economic position that a couple would have. That's just a fact.
Even if you can find me an example of a single parent who is better off financially than an average couple, it's easy to make a strong argument that their child would still probably have a better chance in life if there was a second, loving parent around because the child would receive more emotional resource, love and attention.
There is also the issue of role modelling and the importance of positive male role models for boys in their formative years.
As I have already caveated, these points are only true if the second parent is not abusive. Obviously the downsides of living in an abusive house outweight all of this and in that situation a child is better off with a single parent. However that is very much a 'least worst' solution and not one that should be actively sought after unless there is no other choice.