The fact of their intimate relationship and the nature of that relationship is none of my business. The fact and the nature of *our* relationship is.
Sometimes a facet of their relationship may cause me discomfort or even pain. I may or may not choose to pitch a fit about it or to stay silent. Either way, I will deal with my pain or discomfort.
If I do pitch a fit, then he will have the choice to deal with that or not. However, “not dealing with it” should not entail preemptive dishonesty, whether outright lying or misdirection or deception of any sort.
I feel patronized when my choice to be upset or not is removed by his not being completely open and honest. Lies of omission are as serious as lies of commission.
We have promised to try to have “no twisted knickers” and I have tried to be as open and honest as I can. Not “as much truth as the relationship can bear”, but rather as much truth as we can find.