Within romance, some of the best dark romance novels I've read don't have sex in them. Some of the best dark romance novels I've read do have sex in them. All of the best dark romance novels I've read have stunning characters, plots and world building.

Whether you're writing romance or not, if sex forwards the plot or characterization, use it, but don't ever force it. A reader will spot that off at fifty paces. Writing a sex scene takes skill.