Sexually arousing visual stimuli activate the human reward system and trigger sexual behavior. Here we performed event-related fMRI during visual processing of sexual core stimuli to pinpoint a neuronal correlate of sexual preference in humans. To dissociate gender of the stimulus from sexual preference, we studied male and female heterosexual and homosexual volunteers while they viewed sexual and nonsexual control stimuli. In contrast to previous work, we used core single-sex stimuli displaying male and female sexually aroused genitals. Since stimuli lacked any additional contextual information, they evoked no activity related to neuronal processing of faces, gestures or social interactions. Our prediction was that the sexual preference of the observer determines the neuronal responsiveness to pure male or female sexual stimuli in the human reward and motor system. Consistent with our prediction, the ventral striatum and the centromedian thalamus, showed a stronger neuronal response to preferred relative to non-preferred stimuli. Likewise, the ventral premotor cortex which is a key structure for imitative (mirror neurons) and tool-related (canonical neurons) actions showed a bilateral sexual preference-specific activation, suggesting that viewing sexually aroused genitals of the preferred sex triggers action representations of sexual behavior. The neuronal response of the ventral striatum, centromedian thalamus and ventral premotor cortex to preferred sexual stimuli was consistent across all groups. We propose that this invariant response pattern in core regions of the human reward and motor system represents a functional endophenotype for sexual orientation independent of the gender of the observer and gender of the stimulus.