Porn is purely selfish pleasure. Thus, when a person is married, they're used to being selfish about their sexual pleasure, resulting in strife. And when people think about getting married, they realize that would require them to stop being so selfish, since marriages don't last unless you are willing to sacrifice your own desires in order to help your spouse.

So, porn is a cause, or at least, part of a cause.

As for marriage being an economic institution... Not really. Male and female are different. We think differently, our bodies are different, and so on. In most cases, those differences complement each other. So, marriage is the joining of two people into a partnership that lets them cover each others flaws.