Background: The aims of this research were to determine personal differences depending on the reason for regretting or not first vaginal intercourse and its effects on sexual behaviour later on among young adults, and to assess the association between reasons for engaging in first vaginal intercourse and regretting it. Methods: Data were drawn from the 2017 Swiss national survey on youth sexual behaviours among young adults (mean age 26 years) living in Switzerland. Out of the 7142 participants, 4793 (51% females) answered the question ‘Looking back now to the first time you had vaginal intercourse, do you think that…’ with five possible answers: (1) I should not have done it (6.7%); (2) I should have waited longer (7.7%); (3) I should have done it earlier (7.4%); (4) It was about the right time (67.9%); and (5) I don’t know (10.3%). The five groups were compared on sociodemographic and sex behaviour-related variables, analysed separately by gender. Results: One-third of participants regretted their first experience. In the multivariate analysis, compared with the ‘right time’ group, all other groups were more likely to find their first experience unpleasant and to have done it with a casual partner. Those in the ‘I should not have done it’ and ‘I should have waited longer’ groups were also more likely to have done it because of external pressure, especially among females. Conclusions: The study results underline the significance to choose the right time and the right partner for first vaginal intercourse and the importance of including partner respect and avoiding external pressure as part of sexual education.