This happened many years ago before I was married. It’s a tale of exhibition, no actual sex.
My girlfriend at the time, now my wife, was selected to be Maid of Honor at a friend’s wedding. The usual horrible bridesmaid’s dresses were purchased. This one was a mauve-colored formal dress with a single spaghetti strap holding up the bodice. My wife-to-be, let’s call her, Teri, hoped to make pillow cases with the material after the wedding (she never did).
The wedding was fine but the trouble began when we got to the reception dinner at a nearby golf club. As Teri was getting out of the car, the single spaghetti strap broke leaving the bodice to the whims of gravity. The top was pretty stiff so no one panicked until she sat down at our assigned seats and the bodice dropped slightly, her B-cup boobies were partially exposed. We decided we should probably try to fix the dress before it got worse, so headed out to find some privacy.