Man, this degenerated into quite the argument. . .
Here's the thing: If we haven't seen the females of a particular species, then here's a new challenge to see if you can deliver, BioWare. The fans want it, it would (essentially) add a depth to the universe you've created, and if handled right (That is, a gender selection option when choosing your character/class) it will please fans.
For lore fanatics: Just because they haven't been seen doesn't mean they don't exist. Garrus has a sister, though we've never seen her. Is she not real? There are plenty of infertile Krogan females, many of which are used as scouts or such (as said in ME2), so they're plausable. Salarian females, by Word of God are rare, but the Dalatrass = 1. So they exist. Drell females haven't been seen, but considering they're not an all-female race like the Asari, it's safe to assume they exist too. In fact, it's safe to assume any race, except the Asari or anything already stated as only being one gender, has males and females. . . AND the N7 Special Ops (your multiplayer team) is supposed to be an unofficial unit made of people who have decided to fight. Why should anyone be left out?
Prothean-lovers: Let's compromise a bit. Protheans, except Javik, are extinct. So yes, continuity-wise, they wouldn't fit. HOWEVER, purely for the sake of player enjoyment, BioWare could reintroduce the Protheans if fans/they wanted it enough. Possibly even through a DLC story that follows the Protheans in their fight against the Reapers, during their time.
Lastly, it really just comes down to freedom of choice, not logic, lore, believability, or anything else. People just want the freedom to choose. I like female characters. Whenever I get the choice, I pick a female. That's why even with the Male Quarian Engineer being introduced, I'll probably still main my female QE. Because I can, and I like to. Also why I've never played a male human anything in the game. IF I had the choice, I'd choose a female Turian, or Drell, or whatever else. That's just me though. None of the other things I mentioned really affect my choice, save for if they're available or not.