Yeah, no one ever talks about it, which means you can count on me to do exactly that. Want to know what it is? It's not saying "I love you." It's not meeting each other's families. It's not even moving in together.
It's when you stop holding in your farts.
One of two things happens to a relationship when one or both of the people in it quits trying to suppress their farts. The relationship is either sentenced to Death Row awaiting execution, or you stay together forever.
Those are basically the only two options when you stop holding in your farts, (although technically, those are the only two options for any relationship anyway, regardless of fart-suppression).
I had a friend in college who was obsessed with this as a crucial relationship milestone and marker. She'd hook up with a guy, and after he left in the morning, she'd come into the common room of our shitty apartment in Providence and announce that she could FINALLY let out all the farts she'd been holding in all night, and how she got the worst night's sleep EVER, because she was so focused on fart suppression.
This was also the same friend who literally blamed too-early-farting on the demise of a three year relationship that involved cohabitation. Her feeling was that she'd been much too liberal with her fart-release from the start, which in turn led to a feeling of familiarity and discomfort (emotional, not abdominal) that pretty much sucked the wind out of their romance, so to speak.
I myself have had the experience--in my much younger years, of course--of innocently getting up to pee, and hearing a dude who was clearly holding in a fart let it go, wrongly believing I was out of earshot and smellshot. Naturally, that does not count as successful fart-suppression, and if you fart too early, that's the end of things. You might as well walk right out at that very moment and never look back. Spoiler alert: I did not marry that guy. In fact, I believe that was the last time we ever saw each other, for obvious reasons.
So the next time you find yourself in a new relationship, or even an old relationship that's beginning to disintegrate, cast your mind back to that first time you farted in front of the other person.
It's a critical crossroads, and it deserves an in-depth analysis, preferably with a therapist or other relationship/mental health professional.