In the book's introduction and a chapter on the vocabulary of love and marriage in ancient and medieval times, Dr. Boswell opens the eyes of anyone who thinks it simple for scholars today to decode terms that arose in very different contexts, when marriages between men and women -- at least at their beginning -- were matters more of family alliance, property and offspring than of romantic love.

He whittles away at all the alternative translations and interpretations of these ceremonies that would preclude a romantic and erotic dimension to the unions being celebrated. His book is an imposing achievement, with texts in Greek, Latin, Old Church Slavonic, Hebrew and Arabic.

Ultimately, however, there is a problem. As Ralph Hexter, a professor of the classics and comparative literature at the University of Colorado, put it, "We don't know what they did in bed."

What they did in bed, however, is a central issue if Dr. Boswell's findings are going to play a part in the debate over recognizing same-sex unions legally or religiously. One suspects that his book would get a very different reception if instead of suggesting that these same-sex unions tacitly were accommodating homosexual relations, he had argued that they were meant to be strictly free of any sexual acts, no matter how profound the emotional attachment of the participants, or whether that strict chastity was sometimes abandoned behind the screen of a "spiritual brotherhood."

Professor Boswell is right in warning that moral or visceral objections to homosexuality may create a tremendous resistance to any interpretations of these unions as condoning explicitly sexual behavior. It is also true that the commitments of those who advocate gay rights, who include Dr. Boswell, may create a readiness to accept those interpretations.

Had those commitments biased Dr. Boswell's 1980 book, "Christianity, Social Tolerance and Homosexuality" (University of Chicago Press), which maintained that Christianity was not originally or inherently hostile to homosexuality? That volume is treated as a definitive text by many people demanding changes in social and religious attitudes toward homosexuality. Among scholars, it is hard even to get agreement about how the book was received in their own ranks.

The mainstream of medievalists and church historians was approving, said Professor Hexter, a longtime friend of and intellectual collaborator with Professor Boswell, who is seriously ill and not available to be interviewed. But James Brundage, a professor of history and law at the University of Kansas, said the response was "interested but skeptical, very skeptical."