Changes in Japanese marriage patterns have coincided with shifting gender relations, yet links among feminism, patriarchal legacies, and remaining unmarried remain underexplored in contemporary society and policymaking. This study analyzes Japanese marriage patterns during 2000–2023 and interprets their relationship with feminism and the transformation of danjo kankei. It applies a qualitative document-based approach supported by descriptive statistics. Secondary data from three official Japanese institutions and literature were analyzed by year and gender. Documents underwent Braun and Clarke’s six-phase thematic analysis, and the resulting themes were interpreted through Eagly’s Social Role Theory. The findings show that marriages declined by 9.7% during 2020–2023, while the crude marriage rate fell from 4.3 to 3.9 per 1,000 population. During 2000–2021, the mean age at first marriage increased by 2.2 years for men and 2.5 years for women. In 2020, the proportion of people who were never married by age 50 reached 28.3% among men and 17.8% among women. Thematic analysis indicates that feminism expanded women’s autonomy in education, employment, reproduction, and marriage. However, the patriarchal legacy of, i.e., unequal domestic responsibilities, employment insecurity, and family costs continued to shape marriage decisions. Remaining unmarried is therefore not a direct consequence of feminism but emerges through interactions between changing gender relations and socioeconomic conditions. This study integrates constructivist feminism, Social Role Theory, and demographic evidence to explain changing marriage patterns and to support policies that respect gender equality, individual autonomy, diverse life choices, and social protection for unmarried older adults.