Why should only Eldar be male/female mixed? Surely once you open up that, then each other army must follow?
Also the earlier point about mixed gender Tyranids is stupid. Tyranids reproduce asexually via cloning, and thus have no sexual dimorphism. You're free to imagine the entire horde is female as a result, it's not like GW modelled a penis on every Hormagaunt so that you couldn't.
The Mechanicus have so little flesh left that you can't even tell if they're human anymore. Sexual dimorphism is again meaningless here. I'm all for mixed gender options, but it doesn't help your argument when you throw in such badly thought-out demands as well.