<human>: Met a nice girl at a show a few days ago. She was nice and all but I've been with my girlfriend for a four years now and I wouldn't want to do anything behind her back. We've sent a few messages back and forth in the past few days and after mentioning that I'd have some free time today it looks like we might hang out for a tiny bit.

I don't think I've mentioned my girlfriend to her, though (it never came up). I'm not even totally sure she's interested in me (I might be a little slow to pick up these signs) but I don't want her to get the wrong idea. I think not letting her know could only make things worse if she *is* interested in me. At the same time, I don't want to sound presumptuous or anything and I don't want to come off as a tease.

I thought of adding her on Facebook, so she'd see that I'm in a relationship, but I'm still afraid that might make her think I was being disingenuous and/or that I was trying to cheat on my gf or something. I know guys like me often get annoyed when a girl "casually" mentions her unknown boyfriend but now I'm on the other side of things.

Any advice on how to keep this person as just a friend?

Can you give me a shortened version of the text?
<bot>: I have a girlfriend. Don't want this other girl to get the wrong idea but don't want to come across as presumptuous.