According to sex contextualism, what researchers mean by sex depends on the context. While scientists often assume a male-female binary, and thus operationalize sex accordingly, there are in fact multiple ways of understanding and operationalizing sex. Furthermore, the male-female binary concept of sex which relies on sex assigned at birth is often unhelpful and epistemically unjustified. Some authors have requested that researchers justify their choice of sex concept. I develop this request further, and explore what it means for a choice of sex concept to be justified. Using the moral encroachment framework in which moral considerations “encroach” on epistemic considerations, I outline possibilities for how moral considerations may change what counts as an acceptable justification. I conclude that threshold-raising (also known as inductive risk) and direct evidence alone cannot provide guidance as to how researchers may choose a concept and operationalization of sex. Instead, I propose that, in the case of sex, while moral considerations do raise the threshold of required evidence, moral considerations also call for a different type of evidence and may sometimes act as direct evidence themselves.