i think we are talking past each other (?)...
"Monogamy is a societal construct because it happens to be right for some people."
it is normal in some societies because it forced upon you by the authority of that society and this authority thinks it is the right way to live.
yes it is possible for people to stay together a life time but it is not the norm in humanity unless it is reinforced by the societal authority itself especially in modern times.
there seldom is such a thing as human nature because mostly humans can develop different living circumstances and have extremley complex social structures and psyches.
human nature is in the majority of cases just used to justify behaviours and laws and rules (rationalization)
for example there are (were) tribes in australia that do not have the concept of family as we know it (relatives- father and mother have a child and are their parents) but very complex constructions where completely unrelated persons can be the father or mother of someone or the sister or daughter or brother or whatever.
Have you been in love? Are you just talking about things you've read somewhere?
If not, are you happy having multiple current relationships, loving one person one night and loving a different one the next? They're happy with you fucking other people too, not getting jealous or wanting you for themselves?
I don't mean to ask personal questions. That's what it comes down to, though. Personal relationships. Talking about nonmonogamy being better and not caring at all about being your baby's father. Sometimes your views change with experience. Sometimes not.