Restrictions of free movement have been proven effective in tackling the spread of COVID-19 disease. However, sensitive populations submitted to longer periods of restrictions may experience detrimental effects in significant areas of their lifestyle, such as sexual activity. This study examines sexual activity during the COVID-19 confinement in Spain. A survey distributed through an institutional social media profile served to collect data, whereas chi-squared tests, t-tests, analyses of variance, and multiple logistic regression analysis were used to assess differences among sample subgroups. A total of 71.3% adults (N = 536) (72.8% female) reported engaging in sexual activity with a weekly average of 2.39 times (SD = 1.80), with significant differences favoring males, middle age, married/in a domestic relationship (p < 0.001), employed (p < 0.005), medium-high annual household income, living outside the Iberian Peninsula, and smoking and alcohol consumption. Analyses adjusted for the complete set of control variables showed significant odds for a lower prevalence of weekly sexual activity in women (OR = 0.44, 95% CI 0.27-0.72). Interventions to promote sexual activity in confined Spanish adults may focus on groups with lower sexual activity.