Ok, so on one hand yes, obvious joke is obvious. On the other hand, obvious joke is also pretty offensive. Despite what you may think, there is actually a pretty large girl gamer population. I know at least 3 whom I play League with. I also go to school for game programming and know a bunch of girls there. To top it off, thee are enough girl gamers in one city alone to have their own convention.
Just because there aren't as many female gamers as male one doesn't mean they aren't there, and it is frequent jokes like this that lead to a really bad attitude towards them, to the point where I know plenty of girls who aren't comfortable admitting they are female online because of reactions like this. It goes beyond that, there is a lot of socially offensive material directed at females on the internet and stuff like this really doesn't help.
(Also just in case people think that I am trying to defend myself or anything just throwing it out there that yes, I am a guy.)