Unfaithfulness is not necessarily physical. There is a kind of mental or spiritual adultery which can damage all three people concerned. Hard as it is to forgive physical unfaithfulness, it is equally hard, and sometimes harder, to forgive an apparently innocent friendship between one partner and a third person if it creates a sense of exclusion and deprivation, and destroys the confidence, respect and affection promised in marriage.

Towards a Quaker view of sex, 1964