Yes, that's happened to me too. I think that someone who is already in a relationship is less nervous about talking to me. They aren't making a move on me and I know that, so there's not that fear of being rejected that a single chap might have, so we just talk away and get to know each other and unfortunately I've got a bit too close to a couple of guys, but nothing happened.
When I realised there was a spark I started spending less time with them so that they didn't get the wrong idea and so that I could knock my own feelings on the head and not encourage anything. The feelings went away after a while. I still like these guys, but I have no romantic feeling for them.