[Women] are instinctively, when uncorrupt, monogamous. Men are not…..No good pretending. Men just ain’t, not by their animal nature. Monogamy (although it has long been fundamental to our inherited ideas) is for us men a piece of ‘revealed’ ethic, according to faith and not to the flesh.

TIL Tolkien used the word "ain't"

Unless this was a paraphrase?