It absolutely can be texture. Have you seen a History of Violence? The film has two sex scenes, and they're crucial to the tone of the film, the characterization of the protagonist, and the narrative tension of a marriage in crisis. Would the film work without those scenes? Maybe, but it's a much tougher sell to understand Viggo Mortensen's character and emotional journey without them.

As for fat and ugly people not having sex scenes, that's a selection bias. Most celebrities are not fat or ugly. The few celebrities that are may also be self conscious and unwilling to do such scenes. But they definitely happen. Off the top of my head, Kathy Bates and Woody Harrelson have both done sex scenes despite not being conventionally attractive.