As I wrote to someone else:

I don't want to explain it, because it really was a disaster. But not of a guessable sort. It didn't involve injury, STDs, coercion, cheating, awkwardness, getting caught, pregnancy, Satanic rituals, illegal behaviors, Nickelback, Kevin, Donald Trump, excrement, BDSM, Mr. Rogers, your mom, or anything else I could have thought up before she explained it. It was awful in an eldritch, multi-dimensional sort of way, not meant to be thought of by fragile corporeal beings such as ourselves.

In short, I see why they agreed to pretend it had never happened. PS: I know I probably shouldn't have written an answer if I didn't want to explain, but it was the only secret I could think of.