I've had people friend me after one game, and I can only assume there is one reason why. As an older female gamer, this doesn't bother me. It's part of life. If you started stalking me on XBL, that would bother me a lot. But if you happened to be playing the same game as me, and politely asked me in a message to join, I'm going to let you join.
I've already found love, so it wouldn't be an option for anyone meeting me online, but theoretically, if I was playing with people online when I was 18, I wouldn't say I could never meet someone online who I didn't have a connection with. But it would take longer to happen online, since I don't trust most people are who they say they are online.
I don't think it's impossible, it's just harder. Basically, you are saying to a girl, "Ooh! You like games! Good enough for me!" if you hit on them after a few minutes of knowing them on XBL. When the truth is, there is a lot more to a person than just one aspect. Even I, who am obsessed with video games, wouldn't want to be with someone just because they played games. I am lucky that my fella also loves games, but we also have tons of other things in common.
My best advice, before you start romancing, just talk. Get to know the person and let them get to know you. That will give you a much better shot at actually making a connection if you find the other person has more things in common with you than just games.
And don't lie. Eventually, you'll want to meet this person. If you lie, then you are setting yourself up for disaster.