1.It would be embarrassing if she were better at video games than you. Here you envisioned long co-op jaunts crusading against alien invasions, teaching her to play Portal by romantically holding her hands over the controller, and getting cozy while sniping from a bunker in Call of Duty, but here she is, curb-stomping your face over and over in Gears of War multiplayer. You don't even have to wait to wear that pink sweater she bought you for your birthday to be emasculated.
2. If you are better at video games than her, you will fight every time you give in to her pleas to play together. There are plenty of girls out there that play games. But playing does not imply winning. Ever. Even girls that identify themselves as gamers can be totally inept at video games. She may love playing the easy level campaigns at a nice slow pace, but when you catch her at every spawn point on the multiplayer board and snipe her before she even moves, she'll start to cry. For some reason.
3. You will never, ever get to play single player campaign again. It'll always be "oh, we never get to do anything together, let's play co-op" and then she'll insist on using Convenant weapons because "they are purple and pretty."
4. She will always be misled by beginner's luck on beat-em-ups. Girls don't understand the relationship of button-mashing with beat-em-ups; that is, if you do it, you win. A few button-mash wins will cause them to think they're skilled players, but once she starts trying to learn the moves, you'll crush her every time, and she'll cry again (see #2).
5. She does it for the dudes. Maybe this girl actually likes to play video games. And maybe someday I'll live in a bouncy castle and eat hamburgers for every meal. But she does it mainly because guys think it's cool, and is actually terrible at video games (see #2 and #4). This girl probably also "likes" football, baseball, basketball, paintball, beer, violent movies involving zombies, and firearms, but knows absolutely nothing about them.
6. She says she's a gamer, but what she means is about ten years ago, she used to make a Sim of herself WooHoo with a Sim of her crush, and then set his Sim house on fire. This girl is not actually a gamer, and is, in all likelihood, insane.
7. She says she's a gamer, but the only games she plays are on PopCap and other websites.Not only will she waste your time by getting you addicted to some niblet of a game like Kitten Cannon, she actually thinks this is gaming, which means she's just a terribly confused human being.
8. She'll really put the RPG in MMORPG. Running around as a naked night elf damsel-in-distress who's had her armor stolen and getting attention from the Stormwind sausagefest will be all well and good in the game, but ask her to do it in the bedroom and then there's just something WRONG with you.
9. If your girlfriend is playing video games with you, there is no one to bring you sandwiches and energy drinks. And if she stays the night and you both are awake until 3 AM playing Call of Duty, there is no one to wake up and make pancakes. Already you're out two meals. Do you want to starve to death?
10. If your girl gamer fits none of these descriptions, that means she is probably also a supermodel and a rocket scientist, and is therefore a perfect human being. You do not deserve her and will eventually break up, but video games and sex will be inextricably linked in your mind forever after all those games of Strip TF2, and you won't be able to turn on your console without getting a boner. Happy gaming!