Well…
I have already concluded that internet culture, porn culture, and fandom culture would all prefer a shocking, controversial relationship to a normal husband/wife relationship in media. This is what leads to things such as the "het is ew" trope and the abundance of same-sex pairings in fanfics. There are many who might consider Cadence and Shining Armor's relationship boring if not just plain offensive. A "forbidden love" incest relationship between siblings, however, is just the sort of thing those same people would love to fantasize about.