So in other words, if you’re stuck with someone who won’t fulfill you sexually at all (goes on a strike, as opposed to indulging in a new kink), and who you’ve brought the subject up with and yet won’t let you “go outside” for fulfillment nor make the attempt, and breaking up with said person would harm others besides the two of you (i.e., children, other family members), and sex is Just That Important to you… Then it’s OK to do some aftermarket shopping?
I think you’re going well outside the bounds of the scenario in the OP (we’re talking about A originally indulging in “a kink” with Z and not a total withholding of sexual activity from B, nor are there children involved), but even so, I have to say that I can see your point but don’t agree with it. If my partner were not a jerk, that partner would make one of two concessions – some attempt at fulfillment or a release to find it elsewhere. I did not marry a jerk, nor would I marry a jerk, nor would I necessarily want to stay married to a jerk without first pointing out that s/he is being a jerk in the hopes that jerkitude is curable. Or else, come to realize that gratifying My Particular Missing Fulfillment may not outweigh the cost.
I categorically refuse to be a person who said has looked into someone’s eyes, in front of my family and hers, said words that everyone understands to mean “I will not screw anyone but you”, and then goes ahead and does it.
Yes, that sort of “honor” means that much to me, and it really doesn’t take that much to maintain it. If necessary, I would simply tell my partner explicitly, “fulfill me or I will find fulfillment somehow, and you can’t stop me.”
That doesn’t mean I disagree with you that sometimes it’s kindest to lie to someone. My grandmother was very old when my aunt (her youngest daughter) died of a breast cancer that was diagnosed very late. Per her own request (my aunt’s), we kept the news from her. It was heartbreaking to hear her ask wistfully for her last holiday season why this daughter, alone of her children, hadn’t called to say Happy New Year, and my mother had to come up with excuses as to why she hadn’t called.