Should the earthlings show that they represent a procreating species, this species would be considered dangerous and annihilated by the alien visitors. However the two randomly chosen people are way too shy to perform sexual activity under the scrutiny of the alien scientists and are finally released and put back on to the very train station platform and moment in time, where they previously had been snatched from. While the aliens continue their patrol onward to the next galaxy to be examined for "dangerous" species, the two on the platform decide to go for a drink together and share some thoughts on their embarrassing adventure. They end up in bed and do what almost would have led to mankind's extinction...
What was this story, and who was its author?