Well, if you work in the same office as her, people usually know a little about each others personal lives. Its just common knowledge when you work with the same people for an extended period of time.
Should have flexed your arm for her. 
You're assuming a lot to assign some sort of character flaw here, like I have a wandering eye
what if In sold insurance or cars, and she was a customer. Do I open with "sorry ladies, but I'm taken" . that must be one potent sales technique
