Why is 69 the symbol of the zodiac sign cancer?

The symbol for Cancer is not really the number 69, the numbers are supposed to be ‘lying down’ not ‘standing up’. It’s said to represent breasts as a symbol of the nurturing and protective qualities of the sign. So the circles represent breasts, and the semi-circles are arms, positioned as if supporting a baby.

Actually, the symbol is 96 laterally reversed, if you get what i mean. And it is supposed to be a symbol of a mother, breastfeeding her child.