Abstract Introduction Coronavirus disease (COVID-19) has been declared a global pandemic. In this unprecedented situation, the intimate relationship, sexual behavior, and family functions of partners have also undergone unique changes. There are few reports on whether sexual behavior and family function affect intimate relationships between partners, especially among people aged 18 to 44 years. Aim To analyze the influence of sociodemographic characteristics, family function, and changes in sexual behavior on male-female intimacy, the independent contributions of the aforementioned factors in this population group are required to be further investigated. Methods In the present study, 284 Chinese citizens aged 18-44 years completed the online questionnaire. The univariate analysis and cluster multiple linear regression were used to analyze the associations between sociodemographic factors, sexual-behavior changes, family function, and male-female intimacy. Main Outcome Measure Family adaptation, partnership, growth, affection, resolve (APGAR) Scale and Relationship Assessment Scale were used to evaluate participants' family function and their intimacy. Details of the participants (sociodemographic and sexual factors) were obtained. Results The summary scores, with Relationship Assessment Scale and APGAR scales, were 27.19 ± 4.49 and 6.76 ± 2.28, respectively. About 43.3% of participants reported a decrease in sexual frequency. There were considerable differences among age, education level, sexual desire, sexual satisfaction, quality of sexual life, family function with male-female intimacy (P < .05). The sexual satisfaction and intimacy demonstrated a significant positive correlation (P < .05) by cluster multiple linear regression analysis. Compared with those who had a sexual life of mediocre quality, respondents who experienced a good quality sexual life had relatively higher scores in intimacy. The independent contributions of sociodemographic factors, sexual behavior factors, and family function in male-female intimacy were 13.0%, 38.2%, and 48.8%, respectively. Conclusions Sexual behavior factors and family function were important independent determinants of partner intimacy among people aged 18-44 years. It can provide supportive information for health care to develop intervention plans and services to promote the harmonious development of intimate relationship.