Hi Ana,
In general, it's important for respondents to understand the difference between sex (what we ask) and gender (what we don't ask, currently). These concepts are often conflated, but they are not the same. Sex (male or female) is a complex biological concept that deals with anatomy, hormones, and genetics. Gender is a social construct that ascribes certain behaviors and other cultural norms to the terms "masculine" and "feminine." "Non-binary" is a term that deals with gender identity and those concepts of masculinity and femininity (the "binary"). Non-binary does not relate to sex--people who are male can have a feminine gender identity, a masculine identity, an identity somewhere in-between or completely distinct from the two, or a more fluid identity. Same for female.
I'm not sure if you are actually referencing "non-binary" or if your question is more general about persons who don't identify as distinctly male or female. The term "intersex" describes a constellation of conditions that results in persons who are not clearly biologically male or female. Currently, we do not have a way for respondents to indicate that they are intersex on the ACS (or any other Census Bureau surveys or the Census), but it is on our list of potential future topics for research. Our current guidance is that respondents should respond either male or female based on their best current identification of their sex (not gender). Respondents are free to respond however they wish based on how they currently identify. The sex question is designed--like most ACS questions--to measure the current state, not a state at some point in the past.
Respondents should know that they should not mark both the male and female boxes on the ACS. This will result in missing data, which means the Census Bureau will assign an answer for them based on other information we have, the same as for any other question. As far as how assignment and imputation for sex works, this is not a simple answer. The procedure for assigning or imputing sex when missing is intertwined with age and relationship. However, in general, we can say that we do our best to use other information provided for the household to fill in missing data for sex. Examples include first name, which is highly correlated with sex, or relationship. Our research indicates that sex is one of the most accurately imputed variables on the ACS, but imputation is still no substitute for an answer provided by a respondent.
I hope this helps!
Best,
Caleb