 The definition sounds right: what else is a sexual act if not one that satisfies or “tends to” satisfy sexual desire? But it faces counter-examples. A prostitute performing fellatio on a man does it (typically) not to satisfy or fulfill her sexual desire, but to make money. Nor does the act tend to fulfill her desire, for she might have none to be fulfilled. Thus satisfying sexual desire is not necessary for an activity to be sexual. Whether it is sufficient depends on what we mean by “satisfaction” or “fulfillment”. If it means “the desire is no longer felt for the time being” or “the desire is gone”, satisfying sexual desire would not be sufficient. Taking a cold shower, a powerful sleeping pill, or even just focusing on something else might get rid of the sexual desire, yet these activities are not sexual. If “satisfying sexual desire” means, “the desire achieves its goal” (which is sexual fulfillment), satisfying sexual desire by a particular activity would be sufficient for that activity to be sexual, but the definition becomes circular. The criterion of sexual desire, then, does not succeed in defining “sexual act”.