How does "Porphyria's Lover" transgress from the "prudish" victorian era?
Porphyria's Lover transgresses and contravenes immensely from the "prudish" Victorian era in that it broke with every canon of spirituality, aesthetics, and social norm accepted at the time.
The Victorian era emphasized morality, Christian values, and social awareness. This does not mean that it was dully practiced. In fact, writers such as Shaw, Wilde, Dickens, Austen, and Swift criticized this "fever" of high morality in their comedies of manners, showing how the middle and upper classes were just preaching and not practicing.
Likewise, Browining shows in Porphyria's Lover a number of motifs and themes which would have made Victorian society cringe: Murder, possibly sex (we are not sure that Porphyria wasn't a prostitute lured into the protagonist's lodgins), lust, necrophilia, torture, sociopathic and narcissistic behavior. All these elements showed a side of the human being which is both abnormal and plain sinful (according to the statutes of the time). They were fully exposed in the poem, making the modern reader extrapolate these topics as a product of psycho behavior that would have made headlines in the 19th century.
Yet, it did not. It is reported that the poem went by and was not point of study until much later on in the 20th century when the fields of psychology and psychiatry separated themselves from the general medical practice and focused on human behavior solely rather than making it a part of a systemic process.