Voted yes, but it's rarely black and white. In the case of the thread referenced, A & B were teenagers who had been together a few months. In that situation A & B should be exploring what life has to offer and not tying themselves down to each other. Not saying they shouldn't be together in the long term - maybe it will turn out that way if they're supposed to be together. That's great. But honestly, life is too unpredictable (and wonderfully so) at that age. Go with the flow.
However, if A & B were, say, late 20's, living together, had a dog, and clearly in a position to be committed, then A & B should not be messing around and C (assuming they know the situation) should stay well away. But even then....even if A & B were married....stuff happens....you can't help who you fall for.
In other words, I don't believe in a definitive YES or NO. Life isn't like that.
However, if A & B were, say, late 20's, living together, had a dog, and clearly in a position to be committed, then A & B should not be messing around and C (assuming they know the situation) should stay well away. But even then....even if A & B were married....stuff happens....you can't help who you fall for.
In other words, I don't believe in a definitive YES or NO. Life isn't like that.