I agree. If both people played by those rules, then it could potentially be a non-issue. But I still wouldn't do it, and wouldn't want DH to. But different families work different ways. There is the potential for nothing "bad" to happen in this situation, but there is the potential for sparks to fly as well. Just to be on the safe side, DH and I would choose not to put ourselves in that position. In fact without even talking to me about it, my husband has made that decision before.
It's fine for other people not to make the same choice as us, though

. I am just pointing out that this is not a black and white issue, which is what it sounds like some here seem to think. There are many factors for each person/couple to consider.