system
Given the following post, respond as if you're commenting on Reddit:
human
Why are there only two genders in nature? Why not more?
gpt
First, a semantic issue. Gender is a cultural term, and some cultures do have more than two genders. 

Sexes are the scientific term, of which humans have two, barring exceptions.

As the animal kingdom comes from a common ancestor, two sexes is generally common. However, there are some species which have individuals that reproduce asexually, are hermaphrodites, can change their sex, or are sterile. You may consider this just different variations of the two sexes, which is appropriate. 

When you move away from the animal kingdom though, you can find more "sexes" in bacteria and fungi. A conjugative plasmid separates bacteria into F positive and F negative for that plasmid, and there are more than one such plasmids a bacteria can or cannot have. 

Some Fungi have mating types that are noted as + or -, which are already different from male and female. Some fungi take this a step further and have even more mating types. These mating types are there to prevent a fungi from sexual reproduction with its own mating type.