He acknowledges that the book focuses on male same-sex unions, explaining that the historical evidence from pre-modern Europe predominantly discusses men, the socially dominant gender of the time.

He moves on to look at the evidence for same-sex "paired saints" in early Christianity, such as Nearchos and Polyeuct, Ruth and Naomi, and Serge and Bacchus, arguing that these couples were perhaps romantically involved.

[8] Chapter seven, "The History of Same-Sex Unions in Medieval Europe", looks at further evidence for such ceremonies in the Byzantine Empire, including stories such as those of Nicholas and Basil, and then examines the Christian prohibitions that were later introduced to put a stop to them.

[9] Speculum, the journal of the Medieval Academy of America, published a review by the historian Joan Cadden of Kenyon College, in which she described the book as a monument to Boswell's "prodigious accomplishments", providing an opportunity to celebrate his life and mourn his death.

Speculating that Boswell's arguments will spark debate for decades to come, Kaelber suggested that even if his ideas were rejected by future scholarship, the book would still be very important for showing how "social arrangements and processes can shape and sometimes bend normative perceptions of the boundaries between friendship, affection, and love."

[11] The classicist and critic Daniel Mendelsohn, himself openly gay, published a scathing and detailed review of Boswell's book in the scholarly journal Arion.

[13] According to Mendelsohn, judged as a work of philology Same-Sex Unions in Pre-Modern Europe is a "bad book", and "its arguments are weak, its methods unsound, its conclusions highly questionable".

[14] Mendelsohn argued that Boswell failed to establish his two basic contentions: that adelphopoiesis (literally "creation of brothers") was a ceremony akin to marriage rather than a celebration of a ritualized friendship, probably intended for the reconciliation of heads of households, as argued by previous scholars who had considered the matter (such as Giovanni Tamassia and Paul Koschaker), and that homosexual lovers were commonly characterized in the classical and early medieval worlds as "brothers".

"[15] The sexologists Timothy Perper and Martha Cornog reviewed Same-Sex Unions for The Journal of Sex Research, noting that Boswell was clearly aware of the social repercussions of his work for contemporary lesbian and gay people.

Although stating that they were not convinced by all of Boswell's arguments, and were unqualified to judge many others, they thought that the book constituted a "major work of historiography" by bringing many neglected primary sources to a wider audience.

Praising his use of footnotes, she considered it meticulously researched, and ultimately notes that it "adds both liveliness and dignity to the debate" regarding same-sex unions in pre-modern Europe, as such providing a "fitting memorial" for Boswell.

[22] Briefly dismissing Boswell's work as unsuccessful at placing his interpretations within the "customs, language, and theology" of the time, he urges the reader to read Shaw's review in The New Republic.

Writing in The Christian Century magazine, the historical theologian Philip Lyndon Reynolds expressed "profound problems" with Boswell's positions, which he claims rest largely on "ambiguity and equivocation" and "conceptual slipperiness".

Continuing Boswell's line of research, it served as a defense of his thesis, confirming that, "[f]or a very long period, formal amatory unions, conjugal, elective and indissoluble, between two members of the same sex were made in Europe, publicly recognised and consecrated in churches through Christian ritual.