I can't believe I have to say this. Not every straight line is a penis.

The one looks like it does because it is derived from a tally mark, the simplest mark it is possible to make that differs from no mark at all that can be reliably distinguished from a random imperfection or accidental mark. (A dot is simpler, but not reliable.) Pretty much every writing system uses a line (usually vertical) for tally marks: / is one, // is two, /// is three, etc. Most written number systems derive 1 from that.

The earliest number systems had no zero. Some times they would have a special symbol to represent a lack of any quantity, or a zero baseline for measurements, like the ancient Egyptian nfr which was represented as a stylized heart with trachea attached. The ancient Greeks did not consider zero to be a number, even one (or "unity") was suspect. Neither they nor the Romans had any way of writing zero as a number.

The concept of zero as a number (and not just as a lack of something) is generally accepted to have been invented by Indian mathematicians in the seventh century CE, originally drawn as a large filled dot. That eventually evolved into a circle, better to show the void (zero) within. Nothing to do with lady bits.

From India, the concept spread to Cambodia in the east, and to Persia in the west, from there to the Arabs and finally arrived in Europe in the 11th century via the Spanish Moors. The so-called "Arabic numbers" took another five hundred years to get widely accepted and overturn the use of the old, inefficient Roman number system (which I remind you had no zero). European mathematics was held back for centuries by their use of Roman numerals.

The binary number system itself was not invented until the late 16th or early 17th century.