Before her baby was born, Beth watched an ultrasound picture of the fetus in which the genitals were clearly visible. Beth said, "It's going to be a boy for sure!" Strictly speaking, Beth is referring to the unborn infant's:

sex

Explanation:

Sex is a term that describes the biological and physical traits that differentiates a male from a female. The difference in genitalia and genetic is usually the bases upon which the society uses in differentiating a male or female based on their sex.

For example, as stated in the question above, Beth categorized the unborn infant as a boy based on the clearly visible genitals that she can see through the aid of an ultrasound picture of the fetus. Beth is referring to the unborn infant's sex.