I think it’s probably the future of relationships, just because most non-religious people can’t produce a coherent case for monogamy except “think of the children”, and most people will very reasonably say “well, I’m not planning on having children for a while so I’ll be poly for now”. It also seems like nonstandard relationships getting more accepted is a trend (gays, interracial marriage, etc) so I guess I should bet on the trend continuing. I’m not sure there’s a real dichotomy between “genuinely in love with many people” and “wants to be promiscuous”. For example, I became poly because my girlfriend at the time was poly and it would have been weird to have a mono person in a relationship with a poly person. Then I continued because why not.
It isn’t entirely about logic, it’s about how they feel. I read a while back that something like 30% of one-night stands end in relationships. Many people find casual sex unsatisfying. While poly is not identical, it wouldn’t be surprising if it really just isn’t a fit for most of the population without modifications of some kind.
Not to mention the likelihood of devolving to polygyny with all the bad consequences that entails once it gets normalized for the normies.