This study aimed to identify some of the reasons held by participants for either disclosing or concealing a sexual fantasy. Participants were asked to describe either how their partner responded to a disclosure or (for those reporting on an undisclosed sexual fantasy) how they anticipated that their partner was likely to respond. A mixed-methods approach was employed. Two hundred and eighty-seven participants were surveyed about their reasons for disclosing/concealing fantasies and either their actual or anticipated disclosure experiences. The majority of participants (69.3%) indicated having disclosed a sexual fantasy at some point in their relationship. Five categories of reasons for disclosing/not disclosing a sexual fantasy were generated through descriptive content analysis. These categories were sexual gratification, relationship-motivated, partner traits or characteristics, communication patterns, and specific fantasy content. Participant descriptions of their partner's response to the disclosure of their sexual fantasy were frequently coded as positive. For those describing how they believed their partner would be likely to respond to the disclosure of reported sexual fantasy, these anticipated responses were often coded as negative. The findings of the current study deepen our understanding of sexual fantasy disclosure and highlight some of the reasons held for either disclosing or concealing sexual fantasies. More widely, these findings may have implications for sex education, furthering our understanding of sexual dysfunction and sex/relationship therapy.