It seems like most people don't get what this is really about. Like the harem genre, there are 2 sides to this. One side is the obvious sexual wish fulfilling fantasy, this is what they're usually decried for. But the other side is the desire for family. If you look at something like Isekai Meikyuu de Harem o, at least for the first girl, their relation plays out more like an arranged marriage than anything else. In Death March MC-kun basically refers to his girls in a manner that he sees them more as daughters. The slave angle is usually used as a plot contrivance or to eliminate the possibility of rejection. I can only think of one LN/manga where MC-kun get off on forcing his will on his slaves (and it's R-18).Looks like the next wave after the isekai flood is slavery fantasy.