It had been fashionable with Utilitarians before Sidgwick's time to insist with a good deal of fervour on this point, and to talk as if Utilitarianism could and should produce a new ethical heaven and earth at very short notice. Sidgwick examines with extreme care and subtlety the duty of a Utilitarian living in a society of non-Utilitarians and convinced that certain of the rules of the current morality are out of accord with his principles. He pours buckets of cold water on the reforming fires of such Utilitarians. When all relevant facts are taken into consideration it will scarcely ever be right on Utilitarian grounds for a Utilitarian openly to break or to recommend others to break the rules of morality commonly accepted in his society.
(C. D. Broad, Five Types of Ethical Theory [London and Henley: Routledge & Kegan Paul, 1930], 157)
Note from KBJ: The Sanctity of (Human) Life Doctrine is part of ordinary, everyday, commonsense morality. Peter Singer, an avowed utilitarian, has devoted a great deal of energy to attacking it. Nowhere, to my knowledge, does he argue that undermining this doctrine will promote utility. In fact, it's pretty clear that it will not. It will create a great deal of disutility. (Most of the horrors of the 20th Century were perpetrated by people who rejected the doctrine.) Peter Singer is a radical first and a utilitarian second, which is to say, not a utilitarian at all. Henry Sidgwick was a utilitarian first and a radical second.
