Of course female characters should be included. It's depressing that as gamers we're still having this conversation, depressing that women (not to mention gay, trans, disabled and non-white people) still often feel a sense of exclusion from what is supposed to be a fun hobby. A sandbox game like Arma offers a good way around this through character customisation to give a sense of ownership and an avatar with which the player can more closely identify.
And to those people who try to trivialise the issue (e.g. the argument that it isn't as important as fixing bugs) - I've dealt with matters of equality and diversity on a professional level, and in my experience, symbolic matters like this are very important indeed to the people affected. Women and minority groups do suffer from prejudice and discrimination in their everyday lives and it does have a corrosive impact on both individuals and society.
"It's only a game" - no, it's more than that. It's a piece of popular culture and as such it has a role in shaping society's outlook and attitudes. It can reinforce stereotypes and prejudices and pander to it's "core" audience or it can challenge them. It's time gaming did more of the latter. You won't reach everyone, but you can at least make some of your audience think. It doesn't even need to be overt. The mere inclusion of female soldiers in Arma 3 would be a powerful statement on its own.
From a personal perspective: as a 39-year-old with a tediously grown-up lifestyle, I have enough trouble defending the mere fact that I still play games to non-gamers (because gaming's just for kids, isn't it). Frankly, I'd be embarrassed to show my non-gamer friends and family the neanderthal misogyny on display in this thread, because it would confirm their negative stereotypes about gamers. Some of you might just want to think about that a little.