Bottom line: WHY O WHY were you going out with her in the first place? That's the first mistake. You mentioned you already have a GF, then wth are you doing going out with another woman? Friends and flirting are one thing, it's quite another to go out and get involve even though it seems all innocent to you.
"Wanna go out on a date?"
That, my friend, is like opening a whole can of worms if you don't intend for it to go anywhere. You should have heard the warning light go on when she said 'yes' to your request. Even if you realized your mistake and decide to back down, you should add that 'hey, we can go out together. My GF knows this great place. Let's all meet together.' As in a big group.
In any case, it's probably a good idea to not meet with her. Break off any contact. You can still see her (as in, in the hallway or whatever), a brief 'hi' and 'how are you?' is probably sufficient but I wouldn't go beyond that.
And who gives a fudge what other co-worker think? I wouldn't care about what they think just so long as you make it clear that you've got nothing to hide and that you are her are just aquaintances. (spelling).
In future, DO NOT go out alone with another woman. You already have a GF for crying out loud!
Good luck.
