In reviewing other reviews of this book, it's apparent that those approving of it / objecting to it are mostly people who approve of / object to same-sex marriage. Which tells us little about the book's veracity or validity.
There are no truly neutral observers, simply variations in the degree to which one is prejudiced toward or against a particular position. This book is so heavily footnoted and appendixed, and Boswell seems to be at such pains to clarify exactly what he is and is not claiming, and why, that it is hard to believe he is not being intellectually honest, despite the fact he had a vested interest in promoting same-sex marriage. In short, Boswell gives at least the illusion of objectivity. This is enhanced by the fact that he builds his argument over many chapters, showing the social context into which church-authorized same-sex unions fit, rather than presenting documentation on just the unions -- a point which most of his detractors conveniently overlook.
The bias -- and carelessness -- of some reviewers is blatant. Kevin Davis states "...Boswell argues that rituals for the binding of two males (in Eastern Christendom) between the 12th and 16th centuries is evidence of the support for same-sex marriages in earlier Christianity. This is yet another example of a scholar misinterpreting historical facts in order to serve an agenda."
Anyone who paid attention while reading this book knows that the preceding statement is mostly untrue. (For example, the rituals existed for over 1000 years, and were in use in other parts of Cristendom.) Boswell explicitly states, repeatedly, that these same-sex unions were not, as far as the ceremony itself went, marriage ceremonies (which is why the book has the title it does), and he repeatedly shows how they differ (though the difference is not huge). He does, however, draw a distinction between what is written in a ceremony, and how people perceive the ceremony, suggesting that "the populace" might have viewed the union as a marriage (though not necessarily with a sexual element).
"matt" states "I would suggest that we all need to be careful in reading into texts and history what would make us feel better about ourselves." Agreed. But what about reading text and history based on what we currently perceive as true or false, right or wrong? matt conveniently forgets that "the church" systematically persecuted homosexual men and women for a thousand years -- and he's surprised when some of them are happy to find a bit of history that indicates the church at one time supported (if probably only unintentionally) their affectional preferences?
Which brings us to the issue of the essentialist / constructivist argument. Throughout the book, Boswell (it seems to me) leans in the constructivist direction, by attempting to interpret everything in the context of how the people of the time would have seen or valued it. This is far from trying to force a "modern" homo / hetero perspective on the analysis, which many critics seem to accuse Boswell of doing. (They, of course, do the same thing, but from "the other side".)
Those disagreeing with Boswell do so primarily by grossly misreading him, by taking his arguments out of context, or out of simple prejudice. They don't want to believe his interpretation might be correct.
There is another set of "facts" not discussed (or even mentioned) in this book. One is that homosexual practice between consenting adult males (I'm deliberately omitting paederasty, the love of young men, violation of slaves & prisoners, etc) is not unheard-of historically (qv, the pagan Celts). A berdache was often married to a man of the tribe (see Ruth Benedict), who presumably enjoyed sodomizing another male.
I don't believe Boswell is guilty of stirring up a hornet's next by means of bad scholarship or specious reasoning. He doesn't seem to be indulging in either.
As (the non-gay) friendly Professor Peter Schickele likes to say... "Truth is truth. You can't have opinions about truth." Boswell's interpretation of the historical evidence is almost certainly correct.