Abstract Introduction Many of the sexual problems of the couples that come to psychology and sexology consultations are due to erroneous beliefs surrounding the topic of sex. Beliefs that have been turned into widespread sexual myths and fallacies in society. In this study, we conducted a survey on knowledge and fallacies related to sexual intercourse with a large Spanish random sample. Our aim was to evaluate the presence of these fallacies and myths in today's modern society. Methods A total of 1162 participants of different genders (female, male, and non-binary), different sexual orientation (mostly heterosexual) and different ages (from 14 –two cases- to 64 with mean of 24.62 and SD of 8.96), answered an online questionnaire, developed ad hoc for this study, during the years 2019, 2020 and 2021. This questionnaire or survey collects the different fallacies, myths and “machismo” attitudes collected in clinical practice (psychology/sexology) that have caused couples or individuals to request professional therapeutic help, as well as knowledge related to sexual practices. Results The results show that the current level of knowledge is in general good (between 99 and 73% of adequate responses, with some exceptions mainly related to anatomy), higher among women (reaching their highest percentage of correct answers at 99%) than men (reaching their highest percentage of correct answers at 97%), and independent of age. In addition, we observe the weight and relationship that certain myths and traditional cultural fallacies have, as well as the presence of false expectations and beliefs about what the "ideal" sexual relationship should be, which are still very present in modern Spanish society, and which are responsible for the majority of clinical consultations and alleged disorders related to couples’ sexual life. Conclusions These myths, false beliefs, and unrealistic expectations, as well as the subtle attitudes of "machismo" (that people are not aware that they possess), must be combated (information, campaigns) since they are responsible for many of the apparent disorders and dysfunctions in the human sexual response (which are mostly attributed to women), and in the sexual life, in general, of couples. Policy Implications This study presents the level that these fallacies and negative attitudes are present in a large sample in Spain, and the questionnaire developed will allow evaluating the level of presence of these fallacies and negative attitudes, both in different societies/ groups and in particular individuals. Information that will be of great help to professional therapists who treat dysfunctions and sexual problems in couple/marital life, which are based on these fallacies, on attitudes of “machismo” and/or on the lack of knowledge on the one's own sex and that of their partner.