 The most famous contemporary philosophical account is Thomas Nagel’s psychologically-based view of sexual perversion. “Natural” sexual desire involves a multi-leveled mutual awareness by two people of each other: X perceives sexual excitement in Y, Y perceives excitement in X, X perceives that Y is excited by X, and so on (1969: 10–12). Sexual desire is complex in that it includes X’s sexual arousal by Y and X’s feeling sexual because of Y’s arousal by X, and so on with higher levels. Sexual perversions are then standing preferences for sexual activity that does not involve such multi-levels of sexual arousal. Since this view locates naturalness and perversion in the agent’s preferences, the sexual act itself need not mirror this structure; only the desires need have this complexity. Thus, it is inaccurate to accuse it of being sexless (Solomon 1974: 336) or to evaluate it by giving examples of non-complex sexual acts (Kupfer 2016: 333).