I’m not sure why you assumed she would go and on about it. Were you already in relationship, did you know her quite well, did she already behave like that before? Sometimes we may have prejudice and fear reaction from another person just because we saw similar people do the same before, but every person is unique in his ways, and your girl should be the most special above all else. What if she just wanted to know the truth from your mouth, what if she wanted to be reassured that your behavior was all right, that you actually knew what was wrong about it? If someone makes plan, and builds castle upon a fact or the absence of it, shouldn’t she be informed so that she can make the best out of her work for you both?
If you saw no wrong in it and she does, that may be a sign things may not work in the long run, you’ll always be tempted to do those things she considers wrong, she may eventually find out, be really shocked and angry. Why shouldn’t you look for someone who approves you for all you are? Why shouldn’t let the other person look for someone she can fully approve? Is the present worth the pain in the future? I suppose you thought so and she did not. We do not think alike, let’s find out who we are and respect each other. But that’s the fatality of love, when people do not want to part and keep hoping the other may change and see the right of their way.

Anyway the above scenario is a bit different. The omission happens before the relationship starts, when Jerry and Martha are friends and Jerry did not mind what Martha have been doing unless she wanted to be with him. When Martha proposes, Jerry minds the fact, he bases his decision upon that fact and may decide he does not want to stay with Martha at all. Martha denies him the truth and reveals it to him when it is already too late for Jerry to act in such a way to make things right for them both. Martha had decided what was the best course of action for them both all by herself, she did not consider Jerry’s feelings or the importance he put on the facts. Tough, Jerry failed to state would have been the consequences of an omission: that looked so unlikely, he trusted her.

Now, there are things one may find once the relationship starts. All that is fair and good, you cannot know everything in advance. However, shouldn’t be something like that fact in the scenario always be mentioned before the relationship starts? Especially when the other person seems to give importance to it? If something is found during the relationship both people may conclude they are not one for the other, or accept each other limit. For example if your girlfriend really got fixed on that fact, you could put sense into her by telling her to either accept you for what you were or leave you (I know it’s easier said than done for fear of a break-up). However if something is assumed to be true for a long a time, and one day you find out that’s false, wouldn’t you be scared and hurt? What if you put a lot of effort to build something that relied on that foundation, wouldn’t you feel like you’ve taken advantage of? Wouldn’t we be all scared if tomorrow the sun did not shine?