From memory, I can't remember where I first read it, but it was pre-internet. All I can remember was the premise and punchline, the original author (whomever they are) should not be held responsible for anything, other than the original idea -> if it is badly told, the fault is entirely mine.
In the far future, man discovers not just a convenient means of inter-stellar space travel, but the first alien race as advanced as ourselves.
Earth sends it's representative to their planet to learn about them, their science, history, culture, etc. He is assigned a female representative of the planet's most technologically advanced species to aid him in his research.
After a year on the planet this astronaut decides to ask a question that he's not sure will be a taboo subject or not, he asks, "how exactly, does your species reproduce?" She replies, "Perhaps it's better if I just show you…".
Now, this astronaut gets a li'l excited at this prospect because he's not had sex in over a year, and his alien guide is quite attractive. To his dismay, she leads him to her vehicle, which this species just so happens to call a "car".
They drive for hours, finally reaching their destination, a large structure. He notices at once that there is an endless stream of aliens leaving by one of the structure's doors.
She takes him to the other end of the structure and they enter. "This is one of many lines in this structure, there are many of these structures on our planet". As they pass into the first area, he's astonished to see an assembly line and a machine that places a pair of feet on the conveyor. As they walk the length of the conveyor, passing from room to room; legs are added in sections, then a torso and finally the head. The alien steps off, dresses and leaves the last room to join the other new aliens filing out the other door. Our hero is speechless.
As they drive back to his accommodations, she notes his amazed state and asks, "Why the look of astonishment, how does your species reproduce?" His look of amazement changes into a leer (hey, it's been over a year, right?), and he replies… "Perhaps it's better if I just show you…"
When they reach his place, he leads her into the bedroom, takes off his clothes, then hers and proceeds to display all his love-making skill that he can remember, …after an hour they both come to a climax, and the alien starts laughing hysterically and says, "that's it?"
Taken aback, our hero asks, "Wasn't it good for you?"
She replies, "It was fantastic, …no it's not that you performed badly, it's just that this is how we make cars."