Hi Anonymous,
You make a valid point that if you are socially isolated engaging in activities that allow you to absent yourself from social contact can be a problem, and these findings from college students may or not transfer over to married adults in the same way. However, the research described here is really focusing on people who lack a shared social network with their partner (i.e., couples who do not share friends), not people who lack a social network entirely. Married couples are likely to be less prone to this lack of shared friends than younger dating couples, as they may have in-laws in common or their own children, and they usually live together, so they would know the same neighbors. So I would expect that married couples would be less likely to benefit from sharing media, not because they're isolated, but rather because they already have a shared social network and don't need to compensate for it by bonding over TV shows and movies.