This is certainly where the modern religious justification comes from, but it was commonly prohibited except in extreme cases or for adultery among many peoples

This is certainly where the modern religious justification comes from, but it was commonly prohibited except in extreme cases or for adultery among many peoples before Christianity: the early Roman Republic/kingdom, among ancient Germans, etc. Indeed one imagines that the social practices of the early Germanic peoples may have had more influence on 19th century norms than religion- theory is one thing, practice another. In a parallel universe without Christianity we might expect a similar social norm!