My girlfriends coworker- I'd love to shag her

So, I've been with my GF going on 12 yrs. we have not got married but have decid to just keep it as it is. We have lived together for 7 yrs.

Lately we've not been having sex like we used to. I try to get things sparked but she's always too tired or some other excuse. We do still bang buts its much farther and few between.

My dilemma is one of her coworkers has caught my eye. I've wanted to fling with this chick for better part of a year now. I'm fairly certain this girl is attracted to me and in another world where I was not with my GF I'm positive I'd be able to date this girl and share some sexual experiences with her. It's girl is not the type I'd want to marry or anything but just seems like a great girl to have mutual fun lovin sex with. This girl is no quite good friends with my GF but they get along at work and do chat on rare occasions outside work.

I think this coworker girl would get with me now except for my GF. But then again she might anyway. My trouble is twofold.

1) I'm Ina relationship with the same girl for a long time now. It's terrible for me to even be thinking of this. I feel slightly dirty because of it but the male animal in me brings me to my next issue...

2) how can I be certain if I made some flirts and suggestions with the coworker that the coworker would not tell my GF? In other words how do I best try to initiate this if I decided to forego issue #1?

Now, I know many of you will give me the prerequisite 'you are a terrible boyfriend' response. I'm ready for those but what I'm looking for in all honesty is how to flirt with this fine piece of ass coworker to see if the possibility exists but do it in a way that can't come back at me if she's not interested. What I don't want is for this girl to tell my GF that I'm making unacceptable advances at her.

This is a challenge but I think I'm up for it.

So..... Advice?