The psychology of adultery has been falsified by conventional morals, which assume, in monogamous countries, that attraction to one person cannot coexist with a serious affection for another. Everybody knows that this is untrue.

Russell argues that prevailing moral codes in societies organized around monogamy distort (“falsify”) how people actually experience desire and attachment. Conventional morality tends to treat romantic/sexual attraction as exclusive: if one is genuinely devoted to a partner, one should not feel serious attraction to anyone else. Russell rejects this as psychologically unrealistic, insisting that human beings can experience sincere love or deep affection for one person while also feeling powerful attraction to another. The remark fits his broader project of separating sexual ethics from inherited taboo and grounding it instead in honest psychology, personal freedom, and the reduction of needless guilt and hypocrisy.