If Rabeinu Gershon decided to ban polygamy because it became immoral, you’d be right, but that’s certainly not what happened. (Agav, I read that his charomim were a response to men who traveled on business and decided to abandon their wives.)
I understand the need, on occasion, to fit into secular society to some extent, but our inherent sense of morality should not change.
As to your final point, I agree that not everything halachic is devoid of subjectivity, but that doesn’t make it non-halachic, as long as it’s not subject to society’s ever-changing “morals”.