It seems that socializing may be impeded by lots of singles because efforts of simply maintaining a friendship can be more easily mistaken as a romantic advance.

In my estimation the growing number of singles and late marriages could explain partly why people spend less time socializing (it may not be just video games, internet, porn and work).

https://www.theatlas.com/charts/S1xUZO5MQ (Source)

This may also make it harder for friendships between the sexes to form.

Heinrich et al. 2012 (https://doi.org/10.1098/rstb.2011.0290) listed various advantages of monogamy, but they do not seem to consider such benefits (reduced intrasexual competition goes into the same direction though). Here are things they have considered which are also interesting:

  • Monogamy suppresses intrasexual competition and reduces the number of unmarried men, thereby reduces crime rates, including rape, murder, assault, robbery and fraud.
  • By shifting male efforts from seeking wives to paternal investment, normative monogamy increases savings, child investment and economic productivity.
  • By increasing the relatedness within households, monogamy reduces intra-household conflict, leading to lower rates of child neglect, abuse, accidental death and homicide.