There are structural reasons why there are only two sexes.  Even in flowers it's the same.  One sex is compelled to expend resources (few large cessile gametes) and the other gets away with wild proliferation with little cost (many small motile gametes).   In plants, the male is "pollen", it goes everywhere and is not discerning at all.  Females put in far more effort and must be selective due to the cost they put in.  The males are really taking advantage of that, knowing the female will do it so they are free to fuck anything that moves and still have a really good shot of passing on their genes without putting in the effort.

Because there are two sexes there are two genders, evolved to help direct reproduction towards successful mating pairs.  If I couldn't tell a man from a woman, half my sexual encounters would be unfruitful.

If people want to make more genders like the "blue hair" gender, what they are signalling is "fucking me is pointless."  I guess I'm ok with that.  I'd prefer to know up front.