Margaret Farley’s plea for same-sex marriage (“Love shaped and grounded in faith”, 27 September) is weakened by her confusing three things that for clear thinking should be kept apart: marriage, which is a social institution; sex, which is a biological phenomenon; and friendship, which is a stable relationship between individuals that governs how they feel and act in relation to one another. Friendship, in what is traditionally considered its highest form – disinterested concern for another living being – is independent not only of sex but of society; it can exist between individuals of different societies, even between members of different species, even between creatures and God. Every society has customs concerning birth, maturity, reproduction and death. T