Marriage correlates with wealth being built, it is however not necessary for wealth creation. The crucial aspect is whether marriage by itself is the reason why subjects of our analysis are doing better or whether it's an unrelated correclating event, or whether it is caused by a common causal link.

I'd say it's the last option. The reason(s) why certain people enter into a marriage and why they are better of are the causation we are looking for rather than marriage itself being responsible.

People who have stable homes plan for the future, have a solid income and so on are the ones that on average generate more wealth, than people who don't have this mindset. That is however IMHO the reason why they enter into a marriage and not vice versa.