The sex ratio does not imply monogamy. I suspect most mammals have similar sex ratios whether they are monogamous or polygamous.
A good case can be made for polygamy given that between gestation and lactation, human females are only useful (in a genetic sense) ever 3 years or so. Men are always useful.
The Unintelligent Designer could reasonably argue the best fit males should gather the most desirable females into his harem while the rest of social, but less fit guys, fight off the Canadians and provide food for the Lord’s offspring.
Gay marriage is another thing. Gay men obviously should be monogamous. Since they use rental wombs for procreation gay guys have lots of time to make babies using women not selected for straight guys’s harems!
Maybe that is why we have gay guys?
For that matter it may also explain lesbians. With no fixed male partner, the lesbian can use all that sperm form guys who can’t get themselves harems!