A multi-person scenario may intensify the feeling of being wanted: there is so much attention that it becomes impossible to miss. For someone else, watching is the central element. Another person may be drawn to giving up initiative and no longer being responsible for how the scene unfolds—or, conversely, to directing the attention of several people at once.
Taboo can matter too. Breaking a familiar rule creates intensity without becoming a real decision to break that rule. Sometimes the participants’ impersonality is part of the appeal: they function not as specific people but as roles, gazes, hands, or evidence of one’s desirability.
Research on fantasy content does find recurring motives involving power, boundaries, attention, care, and self-esteem. But it does not provide a dictionary in which every plot has one meaning. The same scene can serve different functions even for the same person on different days.
So a fantasy about several people does not have to answer “Who am I missing?” It may answer a very different question: “What feeling do I want more of?” Sometimes that feeling can be brought into partnered sex without a third person—through gaze, roles, language, intensity, or a different distribution of initiative.