I suppose it depends on the circumstances, sometimes there are other factors to take into account. My last boss was always having affairs here there and everywhere, and regularly used high class prostitutes. I am sure his wife knew, and only stayed with him for his money anyway, so it wouldn't have made much difference.
I don't think I could do it though if someone was "happily married" in theory and their partner was oblivious to it, especially if kids were involved. If they get caught and ruin their own life, that's their fault, I would not want to contribute to breaking up a family.
Might be different though if I'd had a few and they seriously pissed me off.
