There is little to add to the previous post as it succinctly places the quote into perspective. However, one must wonder how Wilde's words hold up in modern times in which cheap, pulp fiction has been written with the express purpose of appealing to a reader's prurient interests. Nevertheless, the immoral thoughts that the reader may have are not the fault of the book, per se, but rather the suggestiveness of its author.
When Wilde wrote this epigram as part of his Preface he was writing against a public that had censured his work, in particular this novel. Interestingly, during his trial for homosexuality, this work was used against him as it was thought that it dealt with perverted and immoral relationships. The whole preface discusses the relationship between the work of art, the artist, and the critic, and it is argued that what the critic "reads" in the work of art is more a reflection on the critic himself than any comment on the work or the artist.