Introduction The COVID-19 pandemic is becoming a major hazard to public health worldwide. This is causing a significant impact on life and physical health, as well as on the psychological well-being of the general population. Since the emotional distress and the social restrictions caused by this epidemic, it must be expected that its impact will also extend to sexual health. The purpose of this study, the first including a large sample of the Spanish general population, was to analyze sexual behavior during the 99 days of confinement in Spain (INSIDE Project). Method One thousand four hundred forty-eight Spanish people, between 18 and 60 years old, were evaluated through an online survey during April 2020. The variables analyzed were the physical and social environment during confinement, sexual desire, type of sexual activity, masturbation, sexual intercourse, online sexual activity, general sexual frequency, sexual fantasies, degree of self-control over sexual drive, sexual abuse, general impact of confinement on sexuality, and emotional mood. Results Confinement has affected the sexual life of half of the Spanish population (47.7%), especially women. Those who reported a worsening of their sexual life are almost three times more (37.9%) than those who reported an improvement (14.4%). Conclusions Different factors have been significant predictors of the positive or negative evaluation about the impact of this confinement on sexual life, such as gender, couple life, privacy, stress level, and the perception of confinement as unbearable. Policy Implications These results have important implications for the public health and more especially sexual health of the Spanish population.