Individuals with neurofibromatosis 1 (NF1) may have problems in managing the transition between childhood and adulthood, such as forming a relationship or finding a partner. We aimed to determine the association between NF1 and forming and ending marital or cohabiting relationships by comparing a large Danish population of adults with NF1 with population comparisons. In this population-based cohort study, we compared a population of Danish adults who were hospitalized for or with complications to prior diagnosed NF1 (n = 787) with population comparisons matched on gender and birth year (n = 7787) through nationwide registries with annually updated information on marriage and cohabitation. Discrete-time survival models were used to estimate hazard ratios (HRs) and 95% confidence intervals (CIs) for the formation and termination of relationships, with adjustment for birth year, gender, and somatic and psychiatric comorbidities at entry. Individuals with NF1 were significantly less likely to form a relationship (HR = 0.65; 95% CI: 0.58–0.73), with the lowest association for individuals ≥33 years (HR 0.40; 95% CI: 0.25–0.63) and the highest for those aged 18–20 years (HR 0.82; 95% CI: 0.70–0.96). No significant difference was found for ending relationships (HR 1.00; 95% CI: 0.86–1.16). In conclusion, individuals who were hospitalized for NF1 are less likely to engage in marital or cohabiting relationships than population comparisons and are older when they form their first relationship. Once a relationship has been established, however, couples with a NF1-individual are not at greater risk of ending the relationship.