But I didn't know how to say it tactfully, without offending them if in fact they turned out not to be a couple. The fact that I was one of the people doing the blatantly het thing didn't make it easier. Does anyone have any suggestions as to the etiquette in such situations?
It's somewhat depressing that the situation should exist in the first place though. Even if I was wrong about these particular girls, it's a very likely occurence that a same sex couple, travelling with a group of strangers, would feel obliged to hide the fact that they were together.