The goal of the study was to find out if sex and sexual orientation have an effect on young adults’ sociosexuality. Another goal of this study was to compare how sociosexuality changed in our population from the beginning of the 21st century. The study involved a total of 1252 participants (405 men and 847 women) of Czech and Slovak nationalities aged 17–39 years from three different samples (A, B, C). The participants were divided by a 7-point Kinsey scale into heterosexual and non-heterosexual individuals and their sociosexuality was examined using two questionnaires: Sociosexual Orientation Inventory (SOI) and The Revised Sociosexual Orientation Inventory (SOI-R). The results suggest that regardless of the evaluation time and methods of obtaining a sample, sociosexuality is relatively stable in our population since the beginning of the 21st century, it is generally sexually dimorphic (men have higher sociosexuality but sociosexual behavior is not sexually dimorphic), is dependent on sexual orientation (non-heterosexuals have higher sociosexuality); statistically significant sex and sexual orientation interactions were also found.