I think it's to attract more people to the site. Everyone thinks things are geared towards guys and that girls aren't on the internet (excluding fashion sites and blogging). So girls ask guys make it seem like there are more girls on the site and they are the ones asking the questions cause guys ask questions online all the time and it just feels like only other guys are there to respond.
Or I could be completely wrong in which case who knows.
My best guess on why it's like that is because of the whole "ladies first" thing. I don't really know and it is really entirely up to the website owner, administrator to decide what their domain is called. After all they are the only ones that have the actual control over it.
Even though there are double arrows on the logo, Having Girls first in the name is more how the real world works. Girls have to ask guys about lots of stuff all the time and that's just the natural order of things. (j/k)
Girlsaskguys is more easier to say and remember than guysaskgirls in my opinion. Plus I think it's an alphabetical ordering thing (girls being before guys in an alphabetical sense) that makes it seem better.