@Lithium Most MU* play, like Fifty Shades of Grey, is a fantasy. One of the appeals of fantasy is that it doesn't have to be realistic - a story about a healthy, ordinary BDSM relationship never would have sold as many copies as Fifty Shades, because it was all about the convergence of several different common fantasy scenarios with a bit of spanking and leather sprinkled on top. Calling it abuse, as if actual people were harmed or adult female readers are incapable of distinguishing fantasy from reality, has always bugged me.
Same with the various sexy parings or groupings running around MU*s. Very few of them are "healthy", but they're not meant to be, any more than D&D is meant to be a thoughtful and respectful portrayal of exploration and colonization.