In this dissertation I investigate how individuals group others into sexual orientation (SO) categories based on a target's known sexual behaviors and romantic interests. I hypothesize that individuals known to have any non-heterosexual sexual or romantic interests are more likely to be perceived as "gay" (and not "straight") even when there is clear evidence of heterosexual interests and behaviors as well. This phenomenon has been termed "heterosexual exclusivity" in this work. In the process, I examine relevant writings and research on SO, including works related to SO in history, the conceptualization and measurement of SO, determinants of and influences on SO, the essentialism and social constructionism debate with regard to SO, innate bisexuality, and bisexual erasure. Additionally, I give specific focus to how and why men are affected by, as well as perpetuate heterosexual exclusivity. In doing so, I examine writings and research on the role and construction of masculinity as well as homophobia and the overlap of the two. I hypothesize that adherence to traditional masculinity and increased homophobia are predictive of increased heterosexual exclusivity in men. I also hypothesize that men are more likely to be the primary agents and targets of heterosexual [exclusivity]. The results supported most of these hypotheses. / text