When did I mention that the time was over? Modern history has continued to provide cases of population imbalance.
Most noticeable in the aftermath of World War 1 and World war 2. For example, in the German state of Bavaria, a survey in 1946 found that there were almost two women aged 21-23 for every man of the same age group. The simultaneous 7-12% rise in out-of-wedlock births that year is attributed to this. If German culture had been more accepting of polygamy, it is likely that this would have been avoided.
Even today, in the Northern Marianas Island, there are only 0.77 men per woman. If every man was monogamous and faithful, then nearly a quarter of the women on the islands would either have to live as spinsters or emigrate in search of a life partner.
Mauritania and Bangladesh are Muslim countries where there are 0.9 men per 1 woman aged between 15 and 30. Again, without polygamy and infidelity, this would leave 10% of the women unable to find a life partner within the country.
So, as you can now see, polygamy isn’t advantageous only in a certain, past, era. There continue to be times that arise when its presence in a society would avoid illicit behaviour.