Sex (either of a beholder or an evaluator) is the variable that is one of the most used (or controlled) within the research of the facial attractiveness. The presented study explains the roots of the importance of “sex” variable for this type of research and also discusses the ways how it can be possibly measured. Based on the key characteristics of categorical (nominal) and interval (scale) variables, authors outline the nature of the biological (man vs. woman) and psychological (gender inclination) attitudes towards such a measurement. The study also provides an explanation of the differences and consequences (e.g., for further statistical procedures) of nominal vs. cardinal measurement of “sex” variable. Possible applications of the final conclusions are discussed – e.g. within the attractiveness research.