... why do I have arms on opposite sides of my body? Ditto legs, breasts, eyes, ears etc.
IIRC (A-level Chemistry was many years ago) 'cis' describes a molecule that has groups of atoms branching off it on the same side, rather than opposite sides.
If I'm cis, why don't I have, say, two left arms? Everything that I have pairs off is arranged symmetrically, on each on opposite sides.
Clearly, I must be trans.