Standards are always relative both between and inside any society(ies).  

The Romans before Christianity existed followed some Greek standards, but not others, even though they worshiped the same "Gods".

 The citizens of Rome had different standards than the non-citizens, and the First Families of Rome had even different standards from either of those two groups. 

Even if we go back to a time when Catholicism was the only Western religion, Islam and multiple other religions also existed with their own standards, some of which were followed in Europe by some people.

The standards applied in Catholic Europe varied, say,  between France and Spain.  Standards also were different for the peasants, the nobility and the clergy  in the same country.  Same religion applied to those three classes of people, different strokes for different folks.

Billions of people in the world believe in plural marriage; we are a minority to follow monogamous marriage and have been for centuries.