Or to put it differently,

if 50% of the population is bisexual,

or the gender ratio is distorted,

or overall hormone levels and mating strategies significantly change,

then it makes sense to revisit the idea of multiple marriage for normies.

(Much like, for vegans, vatmeat becoming available means revisiting the idea of eating meat!)

A social synthesis for a healthy society depends on various conditions/truths about that society.  The rules for a society of near-immortals are naturally going to be different from the rules for a society with a life expectancy of 40.