Trigger nailed it. I have a couple examples of this.
You can read back to a post of one of my horror stories when I told my ex girlfriend about my PE (Extender, clamping). We had a horrible break up only a couple weeks later where she left me and drove over to my “best friends” house and the two of them hooked up that night. I posted the mp3 of the phone call she made the next day where she told me how much bigger his dick is than mine “without having to use stretching devices”. So, that reveal to her certainly bit me in the ass as I imagine she told that story to a few people I know.
The girl I was with next was not quite the same terrible experience, but after I told her and showed her what I downplayed as “casual exercising” she one day during a discussion with me told me how she “knew” I had a big mental problem with my penis size. Now, like many on here, we know we are not “small” but just striving to be closer to the “King of the Hill” and I don’t think I have ever felt embarrassed by my size (my erection quality.. Yes, but not the size), so I realized that when telling a girl what you do, it may also get read as major insecurities even if it is not. That in itself can emasculate you somewhat and leave an indelible impression.
Just keep in mind, no matter what happens in the relationship, your PE experience outside of our “AA of Penis” here, may find its way to others in your sphere of family and friends if you tell.