Specific to the current problem, something like, "Mom and Dad, I did not tell you about it because
*I* did not take it seriously;
*I* totally dismissed it.
To me, it was a non-event...and always will be nothing more than that. Getting upset because
*I* did not find it worthy of
my further time and energy won't change
my truth about it. I don't have anything else to say."
And just keep repeating those same sentiments...even if you also have to repeat exactly the same words.
To change the habitual dynamics and ways of relating between the three of you. Your anger and frustration, as valid as those are, won't help your own cause.
A different approach would be to try to put it back on them:
Tell them that you're confused because you cannot reconcile in your own mind what they are requesting/demanding you to reveal about your personal ADULT life versus what you understood were their parenting lessons about appropriate boundaries, etc. Can they please clarify?
Ask them if your perception is accurate, that they are actually feeling entitled to force information from you even when you do not feel comfortable revealing it.
Let them know that you value what they've taught you and role-modeled for you; that you cannot in good faith or conscience 'apologize' for maintaining your own healthy boundaries, and this attitude is in great part due to the way they raised you. Are they not happy with the results of their own parenting efforts?
That is. Start asking them to explain
their behaviour, thoughts, feelings, expectations, etc., to you (more, really, to help them start to become aware of their "stuff".) Bonus is that you probably will also get a better idea of what's really going on for them.
Even if they can't act adult-rational, you still can. Stick to your own truth, and just express it as clearly, kindly and honestly as possible. The current pattern may never change as far as they are concerned...but that does not mean that you can't change the way you do your side of things.
Once you learn how to establish and maintain your boundaries, how to not just give in to their nagging, and how to say 'no' to their requests/demands without feeling guilty, then you will not feel that same level of anger and frustration; you'll not feel cornered, helpless and powerless to do anything but upchuck all your personal-private information.
Best of luck. It won't happen overnight but you certainly can empower yourself in your relationship with your parents...whether or not they approve