I think we can at least answer, why have 2 sexes rather than, 3, 4, or whatever.
Assuming the benefit of sex is to mix up genes with others’ (seems reasonable, as that’s what it does!),
In one generation 2 sexes mixes in 50% others’ genes to 3 sexes mixing in 66%; not a huge difference. In 4 generations, it’s 94% to 99%.
So the benefit of the extra sex isn’t huge, but the cost of getting the third may be (just as the cost of finding one mate can be high, esp. if you’re somebody’s prey and need to both be noticed and not be noticed at the same time).
I think we can at least answer, why have 2 sexes rather than, 3, 4, or whatever.
Assuming the benefit of sex is to mix up genes with others’ (seems reasonable, as that’s what it does!),
In one generation 2 sexes mixes in 50% others’ genes to 3 sexes mixing in 66%; not a huge difference. In 4 generations, it’s 94% to 99%.
So the benefit of the extra sex isn’t huge, but the cost of getting the third may be (just as the cost of finding one mate can be high, esp. if you’re somebody’s prey and need to both be noticed and not be noticed at the same time).