Concerned witnesses are compiling a list of the book titles affected here. Having read many of these books myself, I can assure you that a great many of them have absolutely no sex in them whatsoever. They range from the most utterly un-prurient Victorian novels such as E. M. Forster's Maurice to modern children's books such as Lesléa Newman's Heather Has Two Mommies (click and look! it's been stripped of its sales rank!) to nonfiction such as Shane L. Windmeyer's The Advocate College Guide for LGBT Students, John J. McNeill's The Church and The Homosexual, and Kate Bornstein's Hello Cruel World: 101 Alternatives to Suicide for Teens, Freaks and Other Outlaws.
Amazon's formula for defining books as "adult" appears to be triggered by the specific words "lesbian" and "gay." For example, one edition of Radclyffe Hall's novel The Well of Loneliness was subtitled "A 1920s Classic of Lesbian Fiction." That edition has had its sales rank removed, while editions without the subtitle still have their sales rank. (vashtan correctly points out that the only "sex scene" in The Well of Loneliness consists in its entirety of the words "And that night they were not divided.")
Classifying any book with the words "lesbian" or "gay" in its title as "adult" is like classifying any book with the words "husband" or "wife" in its title as "adult." Exactly like the words "husband" and "wife," the words "lesbian" and "gay" are used to specify the gender of a partner in a romantic relationship. This does not usually entail describing sexual encounters! In fact, unlike the words "husband" and "wife," which tend to imply that the people in question do have sex or at least did have sex at some point or other, the words "lesbian" and "gay" do not even imply that sex has ever happened at all (much as some of us might wish that being lesbian or gay automatically guaranteed sex . . .). Amazon would never in a million years use a formula that automatically classified books with the words "husband" or "wife" in the titles as "adult." If they did, the entire Disney empire and Grimm's Fairy Tales 95% of all children's books everywhere would be classified as "adult."
Amazon's formula indicates that Amazon believes our entire existences as families are unsuitable for children to know about. Many of these books were written specifically for children (such as Lesléa Newman's Heather Has Two Mommies was) or for teenagers. To deem them "adult" and restrict access accordingly is to destroy their entire purpose. Yet Amazon believes the words "lesbian" or "gay" are words children need to be protected from ever hearing.
(Update: Now that there's been a huge Internet uproar about the issue, Amazon has changed its form letter responses to say that they recognize this as being a problem and are working on fixing it. Still no explanation of how the problem happened in the first place, though - unless you count the fact that a former employee named Mike Daisey seems to think he knows what happened.)
Additional links can be found here and here. Thanks to mariness for passing this on.