It's what I've observed from my personal experience.
Usually taken girls are less inhibited and interact (or even flirt) with other guys much more freely as they think & rationalize that it's just harmless and nothing will come from that (as they're taken). Understandably, as a bit of flirting is fun & healthy, and girls love flirting & attention. However, sometimes, one thing leads to another... and that's how some taken girls can go too far (I've been in that boat with them too).
Those other guys (and I've been in their shoes many times) usually are blinded by their egos and think that "wow, I must be so on top of my game today/tonight! I haven't really done anything, yet she's so opening up to me. It's on!"
Yep, same here. Now, sometimes this is done deliberately and planned between the couple. They would go out and get their appetite from other people so that they could come home together and shag each other silly.
I've experienced enough of it so my gut detects taken chicks very quickly.
Just last night, I was opened by this really cute girl and it went very smoothly. She was complimenting me on my dance moves and asked a lot of questions. We danced for a bit, and then the alarm in my gut started ringing, because it was going pretty fast with zero resistance and I hadn't even got any sh!t test. I just knew that she has a BF, so I asked a few indirect questions (basically checking if she goes to certain venues, and if her friend - we have a mutual friend - goes with her) and yep, sure enough, she does.
I'd have no problem with going for that girl, usually, but that time there was a specific reason that makes me not want to cut her bloke's lunch.
Whether chicks like her are looking to branch-swing is for another discussion.