system
Given the following post, respond as if you're commenting on Reddit:
human
Despite the diversity of life, no form has three sexes. Why exactly?
gpt
I took a class on the evolution of reproduction. We actually do see multiple sexes (mating types) in certain species like Physarium (a genus of slime mould) which has >500 sexes (but it still only takes two individuals, who are not the same sex, to make a baby).

The main problem that limits most species to 2 sexes is the issue of mitochondrial inheritance. Basically, when the two gametes fuse, only one of them can donate its mitochondria (if they both did, the cell would die). So, nature's solution was to create two general mating types: Male (DNA only donors) and Female (DNA and mitochondria donors).

In most species with multiple sexes, the mitochondria donor seems to be determined by a "rock-paper-scissors"-type of game.