Biology at large is in the midst of a revolution in our understanding of the determination of phenotypes. Epigenetics has shifted our focus from genetic determinism to ecological origins of gene expression. We argue that this shift should be incorporated into sexual selection, changing the conceptualization of sex from a discrete trait to a developmental reaction norm. “Sex is a reaction norm” implies that the variation within and between the sexes is a result of genetic, epigenetic and environmental influences on developmental plasticity of phenotypes. “Choosy females” and “indiscriminate males” constitute one of the best examples of assumed strict sex differences that are in fact phenotypically plastic in response to environmental, social and internal factors. Here we summarize the empirical evidence, which empiricists have explained with trade-off hypotheses: individuals trade-off energy of reproductive decision-making with diverse, usually unitary factors: predation risk or density or OSR, etc. Gowaty and Hubbell’s (2009) Switch-Point Theorem simplifies and unifies these trade-offs into a single hypothesis and works as an integrative framework, both for reinterpreting earlier findings and as a pointer to new directions for sexual selection research. We conclude that it is time to pay more attention to morphological, physiological, and behavioural phenotypes as developmentally plastic and/or individually flexible.