It could be that the co worker did have some romantic designs on you and was angling for an affair and is now embarrassed that you've called her out on it. Or it might be that she was offended at the idea and has now backed off completely out of fear of being misconstrued any further.

It's impossible to tell exactly what the co worker was thinking and what her intentions were and she may never tell you truthfully but I don't think you've done anything wrong at all based on what you've written. I'd have thought most partners would take your actions as a sign of a healthy strong relationship built on trust and honesty. If it's discussed again I'd perhaps use the feedback you got from the other couple as a reason why you felt the need to say something. Perhaps your partner can talk to them and get their angle on things?

It's tough when new friendships don't work out but try not to feel like you're to blame. You merely reacted to circumstances and did your best to keep the friendship going while protecting your relationship.