'Sex' is probably Latin 'secare' (to cut) — the fundamental division of life into two halves.
Either of the two main categories (male and female) into which humans and most other living things are divided; also, sexual intercourse.
From Proto-Indo-European *sek- ("to cut, to divide") via Latin sexus ("sex, gender, one of the two divisions of humanity or animals"), from secare ("to cut, to divide"). The PIE root *sek- yields Latin secare (to cut), segmentum (a cutting, section), English section, sector, segment, and saw (via Proto-Germanic). The semantic connection is the division or cutting of humanity into two kinds. Latin sexus was