It's a fact. Whilst most humans might have an inclination to 'pair bond' many of us also have a roving eye. At a biological level our most successful survival tactic is not - as the church might tell you - remain faithful and worship a deity in the hope of salvation, it is to breed with many partners.
Whilst I may be in an open relationship, I am not some enlightened being immune from jealousy. I am not above jealousy. Because I, like the rest of the planet, used jealousy to get what I wanted and needed as a child. Jealousy is a highly successful tactic the mind has for seeking security outside ourselves.
In trying to fit their round polyamorous selves into a monogamous square hole, they will ultimately fail and repeat a pattern of guilt and self loathing which contributes further to lower their self esteem. Their obvious failures and misery are compounded by the disgust of their peers, parents and often even their friends.