Sworn brotherhood was the English term for the practice of "Adelphopoiesis", or church blessings of same-sex unions, which was described by the gay Catholic historian John Boswell for the Eastern Christian church. A similar practice existed in the Western church, but is much less familiar. The English gay Catholic historian Alan Bray described this history in his book The Friend.
Unlike John Boswell, who has been criticized by some for drawing an allegedly too close analogy with heterosexual marriage, Bray is careful to make clear that he does not see these unions as an early form of gay marriage. However, among the evidence he provides, are many examples of male couples buried together in shared church graves, in much the same way that tombstones of married couples are seen in countless English churches.
These unions were also much more significant than mere buddy relationships of modern times, carrying both religious and legal significance. Like the rites described by Boswell for the Eastern church, these unions were conducted in church. In law, they conferred a legal bond of kinship between the two families.
For an extended review of "The Friend", see James Davidson at the London Review of Books,"Mr and Mr and Mrs and Mrs" (link title), and the discussion that follows.