This book is well-written, which is why I give it 3 stars; but much of it is well-written nonsense, which is why I give it only 3 stars.
First, it is mis-titled. It is not about opera so much as about opera singers, in particular female opera singers who allegedly appeal especially to gay men, such as Maria Callas (there is a whole section (pp. 134-53) on "the gay cult of Callas") - what must my wife think when I play her records?
Second, where it leaves off the gays-as-super-aesthetes stuff, and attempts to discuss testable hypotheses, it often gets the facts backwards. For example: "Records helped kill opera by limiting the repertoire to a handful of repeated and repeatable chestnuts." (p. 47) The truth is of course the exact opposite. Before records, a handful of operas were performed in a season and every season would include at least a Bohème, a Butterfly, and/or a Carmen - one would be lucky to hear a couple of hundred operas in an entire lifetime even if one lived in one of the few world cities with an opera company. Today I can, as I do, live in the desert and choose from thousands of recorded operas whenever I feel like it, an unprecedented cornucopia of operatic riches. Similarly, Koestenbaum states that "opera virtually died with Puccini" (p. 74). That is true only if you don't count Richard Strauss's Arabella (1933) and Capriccio (1942), Alban Berg's Wozzeck (1925) and Lulu (1937), or all or most of the operas of Hindemith, Weill, Krenek, Stravinsky, Prokofiev, Gershwin, Poulenc, Menotti, Barber and Benjamin Britten. Puccini may represent the end, even the Indian summer, of romantic Italian opera, but scarcely of opera.