Karma and I call it our Happy Healthy Sane rule. Very similar to BU's #2. We both feel it is something only to be used when happy, healthy,sane is not being met. And even then it's with a lot of talking. Even when all hell broke loose with Cricket lying time and time again and putting our health in jepardy, I told Karma what I would like to see, that I was removing myself from the situation, and he could make whatever decision he felt right, but that I would no longer be supportive of them being together.
It was still his choice to stay or go, but as part of our honesty agreement, he needed to know that I felt happy healthy sane wasn't being met for him and in a way for me.
We've both agreed that the only time we are going to flat out veto a relationship is when health and/or sanity is being threatened in a major way.