Considering that the Romans were the ruling power at the time, it wasn’t hard for them to make the rules..
They controlled the population by declaring a long held way of life illegal.
The difference between serial monogamy and plural marriage is that it isn’t always “a different wife each nite of the week”.. you don’t switch wives or unmarry them.. you are still married to every woman you love simultaneously.. the bedroom schedule is determined by everyone.. mostly the women decide what works for THEM.. in “normal” polygyny situations (based on love, not force) women make most of the decisions.. there are more of them therefore they have the power. The man is still the head, but there is strength in numbers.. contrary to the “oppresive patriarchal” vision most feminists fear.
So you won’t have any difficulty demonstrating any decrees the Romans made abolishing the marriage laws of their subject territories?