This kind of fanfiction could potentially explore Hermione's curiosity or it might be used to develop a complex plot around her character's exploration of different aspects of sexuality. But it really depends on the specific story and the author's imagination.
The first appearance of lesbian porn stories online is difficult to determine precisely. It likely occurred gradually as the internet grew and different types of content became available, without a clear-cut beginning.
Society would generally view it as abnormal and against the norms. Most societies uphold the values of monogamous relationships and proper family structures.
This situation is highly unethical within the context of a marriage. Marriage is typically based on monogamy and mutual commitment. Sharing a wife in such a way violates the basic principles of trust and exclusivity in a marital relationship.
The main distinction of porn with a story genre from regular porn is the presence of a coherent narrative. Regular porn often jumps straight to the sexual action, while the story genre tries to add some kind of story framework around it. But in the end, both are adult-oriented and feature explicit sexual content.