@engineer,
I think that long-term partnerships are based on trust. Lust only carries so far.
I may decide to forgive someone who has lied/cheated/whatever but I believe it's my right to make that decision. I'd rather have full knowledge.
I don't think there is any case to be made for not telling partners the truth about cheating/other dangers to the relationship.
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On the flipside, I think partners can agree upfront that sexual infidelity is not a deal-breaker - but I think it has to be done upfront.
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I think you have to respect whatever agreements you made as a couple.