Some aphids do have males and sexually reproducing females at certain times, such as in the fall, so that they can produce eggs that can survive through the winter, but for the most part aphids are a one-gender race. Way to think outside the box, there, guys. I can vouch for that. Like, just look at them. Any two individuals of a species can reproduce together, and in some races all members have female traits all chickens lay eggs and all cows give milk.
Maybe to just have Fun?
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The asari even have a Fantastic Self-Racism against asari who join with other asari. Mothra and Gyaos are both all-female species who reproduce asexually. The thing is though, the view of asari presented to us doesn't actually do anything with it. Romance options vary depending on which Ryder twin you choose to play. The Kaka clan of BlazBlue are at least close to being a genetically engineered one-gender race that reproduce via parthenogenesis. Liam will want to show you something on Eos after you earn his loyalty - visit the planet and choose the romance option when it appears in the dialogue.