This is outer space...in the future. Where does it say that because it doesn't happen on Earth that it can't happen elsewhere in the universe?
Maybe the male gamete evolved to mature from a sperm into an adult on its own. Not sure how mixing of genes would work.
It's basic biology. Some things are universal. Biology and evolution, after all, are constrained by physical laws - which is why it is reasonable to conclude that convergent evolution may exist on alien worlds.
With regards to sex, if a species can produce an egg that develops, whether fertilized or not, into another full organism it can be considered sexually female if that species is not truly asexual.
With regards to the Asari, they have female secondary sex characteristics that apparently are convergent with mammalian secondary sex characteristics. Believable? Probably not. But it is indicative that at one time, like the whiptail example I provided (a perfect example, by the way), there must have been males in the species and now there are not.
I wouldn't be surprised if Bioware was influenced by parthenogenetic species on Earth.
Someone else mentioned seahorses...terrible example. They aren't monogendered nor parthenogenetic.