It's a phrase uttered by those entirely confused by the concept
of their parents having had sex to conceive them. They just
can't fathom it.
It's the answer to existence.
"Why am I here? Is it God? Randomness? Is there a purpose to my
existence? Why? Why? WHY AM I HERE!?!?!"
"Oh, you know why you're here. Your parents they... you know..."
"What, the fuck?"
"Yeah, they had sex."
"The fuck?"
"Yes. The fuck."
WHAT THE FUCK!
And so it began, a lifelong cycle of cognitive dissonance, never
to be fully resolved.