Barrier, or no. And does duration of relationship matter?

HB8 at my office been giving me serious IOI for three weeks. Two trips to the bar (with chaparones) and only my hesitation prevented bringing her home. Tonight was a coworkers birthday, all of us met at the bar and near the end of the night (with her boyfriend 20 feet away) I walked into the bathroom behind her. Despite not progressing very far, I also wasn't kicked out and only left because a coworker from a different job walked in and threw me off. She's dated this schmo for 6 years, he's a tool, but who am I to judge. Yes there is serious opportunity, had I an "A" game the point would be moot. But she's obviously not satisfied and interested. So it's less of a method question and more of an ethical one...when do you say no?