Dijkstra said:
The difference is that it isn't necessarily for the sake of their feeling, is it? If they thought it might be non-trivial to you and hid it, then that is what would be something I think that would be reasonable grounds to not feel as trusting.
That's no different to the conventional white lie. White lies are never entirely for the second party's feelings. They may be partially for their feelings but mostly to spare yourself the consequences of telling the truth. To use a stereotypical example, answering truthfully to the question "Have I gotten fatter?" will not only hurt your partner's self-esteem, but it will probably make them angry at you or maybe less receptive to sex because they are now insecure about their body, and generally cause friction in the relationship. In fact, the bigger a deal it is to your partner I would say the more obligated you are to lie about it, both for their sake and yours. After all, getting fatter and saggier is just part of aging. At the end of the day what matters is that you're both happy.
There's no reason to tell the truth about something that cannot be changed and will only potentially cause upset. Anyone could see that the reason she didn't tell her husband was because she was frightened and insecure that he would stop loving her because of a decision she made in the past. To bring it back to my initial comment: If you
genuinely didn't care, you would immediately forgive them and reassure them that you would never stop loving them over something so trivial. Anyone who says "I have absolutely no issue with what you did, but I resent you because you didn't tell me sooner" is lying to themselves so blatantly that it's amazing they don't realise it. Obviously they do have a problem with it or else they'd forgive the lie, not resent it. Imagine how awful it must be living with the belief that the person you love might stop loving you because of a choice you made when you were young, which you probably regret by now.