Are humans naturally polygamous ?

If human beings are naturally polygamous, then why is there jealousy in relationships? Not only are we jealous, when our SO flirts with other men/women, we are jealous even if we're not in a relationship - for example, when your crush kisses someone else. When people say they want an open relationship, but then start complaining about their partner seeing different men/women - it just goes to show, that while they want polygamy/open relationship, they only want it for themselves, but not for their partner/s.

Secondly, how do you determine, what is natural and what is socially constructed? Just because there are more polygamous societies - from this it does not follow, that this is natural. Maybe polygamy is a social construct and monogamy is truly natural, because monogamy is partly motivated by sexual jealousy and exclusivity, which is more in tune with human nature. Societies based on polygamy are also unfair and generally is reduced to - men get as many wives as they want, but women need to be loyal and exclusive to that one man. This is enforced through violence and coercion.