What is the default sex in nature?
Let’s start here, because I believe this is a foundational question that leads to everything else. It also is where life started, several billion years ago.
The very first organisms - and indeed the vast majority of existing organisms - are single-celled, and do not reproduce sexually at all.
What this means is that they do not have sex in the sense of male or female. They are non-binary.
They are also, strictly speaking, asexual. That is, they do not reproduce by sexual reproduction, but by binary fission. They have no sex cells, they do not mate, they do not engage in anything that qualifies as sex.
This is the default in nature: asexual and non-binary.
In another way of thinking, however, one could say that the default is female. Single-celled organisms are far closer to female in essence than they are to male.
A female organism (once we get to sexual reproduction) contains all the necessary equipment to produce offspring, except some genetic material. This holds for binary fission: the original cell contains everything for reproduction, including all the genetic material.