Marital infidelity remains a significant relational transgression with far-reaching psychological, familial, and societal consequences. Despite extensive empirical research, theoretical explanations linking interpersonal deficits to extramarital inclinations remain fragmented. This paper develops the Interpersonal Vulnerability-to-Infidelity (IVI) Model, integrating Attachment Theory, the Theory of Planned Behaviour, Sexual Script Theory, and Interpersonal Communication Theory. The model identifies eight interpersonal variables as distal predictors operating through three mediating pathways—attachment insecurity, attitude formation, and sexual dissatisfaction—to influence extramarital inclinations. Particular emphasis is placed on married postgraduate students, a population exposed to unique academic and relational stressors. Seven theoretical propositions are advanced to guide future empirical research. The findings indicate that extramarital inclinations are not random but emerge from interacting interpersonal vulnerabilities operating through identifiable psychological mechanisms, particularly within the high-stress context of married postgraduate students. The model provides a coherent, mechanism-based framework that advances theoretical integration and contextual understanding of infidelity risk. It is concluded that interpersonal processes play a central and structured role in shaping extramarital outcomes under conditions of relational and academic strain. Future research should empirically test the proposed model using longitudinal and dyadic designs, while counselling and institutional interventions should target modifiable pathways such as attachment insecurity, maladaptive attitudes, and sexual dissatisfaction.