ITT, a bunch of people who know literally nothing about this subject offering explanations.
The character 零 (“líng”) contains a semantic component (on the top) and a sound component (on the bottom), the semantic component is 雨, meaning rain, and the sound component is 令 “lìng”.
The word initially referred to very light rain and so the character essentially means “the type of rain that sounds like lìng”. For whatever reason the meaning drifted from very light rain towards “barely any” and then “nothing/zero”.
The bottom/top usage is simple, the “zero” is the receiving hole and the “one” is the penetrating appendage, i.e. the submissive versus the dominant partner. That usage is definitely slang, though!