I'm not in total agreement with the rest here. Being there will always be something "hanging over" you both when you meet, I'd try this approach. Next time you guys are together, mention to them how you responded to an add for a couple that you thought were them. But it turned out to be someone else. Make a joke of it and let them know that the contact wasn't for playing, as you would not be comfortible with that. This way it clears the air and lets them know you didn't want to play, while still letting them deny it really was their ad. Win win for everyone
Brett