Extramarital relationships are among the major psychological challenges within family systems, potentially undermining mental health and marital satisfaction. The present study aimed to investigate the structural equation modeling of the tendency toward extramarital relationships based on loneliness, with the mediating role of sexual quality of life, among married women attending family counseling centers in Mashhad. The study employed a descriptive–correlational design using structural equation modeling. The statistical population consisted of all married women referred to family counseling centers in Mashhad in 2024, from which a sample of 300 participants was selected through convenience sampling. Data were collected using the UCLA Loneliness Scale (1980), the Tendency toward Extramarital Relationships Questionnaire, and the Sexual Quality of Life Questionnaire (2002). Data analysis was conducted using AMOS and SPSS through structural equation modeling and Pearson correlation tests. Findings revealed that loneliness had a positive and significant relationship with the tendency toward extramarital relationships (p < 0.001). Moreover, sexual quality of life played a significant mediating role in the relationship between loneliness and the tendency toward extramarital relationships, such that higher sexual quality of life reduced the negative effect of loneliness on extramarital tendencies. The model fit indices indicated an acceptable fit of the proposed model. These findings highlight the importance of reducing loneliness and enhancing sexual quality of life as effective strategies to mitigate extramarital tendencies. Accordingly, psychological and counseling interventions focusing on improving marital and sexual relationships are recommended.