What are you getting yourself into, playing games with a date?!
If you always win, she'll feel inferior and you won't get any.
If you let her win, she'll think you're a wuss and you won't get any.
If you try to keep it 50/50, you might actually lose more than her, and then she'll really think you're a wuss, and you won't get any.
If it does end up being 50/50 she'll think you're one of those sensitive guys trying to keep everything fair and even for both sexes, and you still won't get any!
Any way you slice it, you're not getting any playing video games.