At first glance, BDSM (bondage, discipline, sadism, and masochism) may seem like an abusive practice carried out only between heartless, psychologically disturbed sadists and victims with low self-worth. Appearances, however, are often misleading, and with BDSM, this misunderstanding is especially profound, as is obvious in the new movie release, “Fifty Shades Darker,” an erotic romance film.…