Marriage in Iranian society—particularly among young people—is no longer perceived as a self-evident and pre-defined milestone in the life course and is increasingly marked by patterns of delay and suspension. This study aims to understand the social and cultural processes underlying this transformation and to explain the logic of action among individuals who postpone or avoid marriage. The research was conducted using a qualitative approach and grounded theory methodology. Data were collected through in-depth semi-structured interviews with single adults aged 32 and above in Tehran and analyzed using open, axial, and selective coding. The analysis led to the identification of the core phenomenon of “Choosing Singlehood in the Narrow Passage of Safety: The Redefinition of Marriage as a Conditional and Risk-Laden Choice.” This process reflects a shift in which marriage is redefined from a biological and normative imperative to a conditional, calculative, and risk-laden decision. The causal conditions shaping this phenomenon include the individualization of values, transformations in the emotional economy of marriage, a crisis of meaning surrounding the institution of marriage, negative experiences of close others, and structural insecurities. In response to these conditions, actors adopt strategies such as the rational suspension of marriage, excessive evaluative scrutiny in partner selection, prioritization of self-realization, and the conscious acceptance of singlehood. The consequences of this process include structural delays in marriage, an increased likelihood of permanent singleness, strengthened individuality and self-reliance, reduced inclination toward childbearing, and a dual experience of satisfaction and loneliness. The findings indicate that the decision to remain single is not merely a passive reaction to economic constraints, but rather the outcome of an institutional transformation in the meaning of marriage and the growing legitimacy of the single lifestyle.