Many animals species require two members — a male and a female — to sexually reproduce. Why has nature (or the process of evolution) chosen to favor the form of sexual reproduction which requires two members? Why didn't nature encourage the form of sexual reproduction which involved three members?
The reason for sexual reproduction being a better choice over asexual reproduction is justified by saying that sexual reproduction involves mixing of genetic information from two living beings and hence if one set of them had an error, the other would compensate for it. With the same kind of reasoning, we can argue that sexual reproduction involving more than two organisms coming together would cause better genetic variation, shuffling of mutations, etc. and thereby produce a better offspring.
However, most of the organisms as we see today have evolved to reproduce sexually using two beings of the same species.
Consider an animal species which has 24 pairs of chromosomes.
Sexual Reproduction Type 1:
There are two sexes: male and female. During the event of sexual reproduction, the male transfers 24 chromosomes to the female where the chromosomes from both the sexes are mixed to produce an offspring.
Sexual Reproduction Type 2:
There are three sexes: male, female and [insert word]. During the event of sexual reproduction, each member transfers 16 chromosomes to the female where it gets mixed to produce an offspring.
We don't observe sexual reproduction of type 2 (or it is rare). Doesn't the type-2 form of sexual reproduction cause better mixing of genetic information than type-1 form of sexual reproduction? Despite the aforementioned advantage, nature has chosen to encourage the type-1 form of sexual reproduction. Why is the type-1 form of sexual reproduction more common than the type-2 form of sexual reproduction?
This question can be extended to "Why has evolution favored sexual reproduction involving two mates than sexual reproduction involving $n \space (n > 2)$ mates?