A man stumbles into a wind that blows through time and ends up a hundred years in the future. He finds that monogamy has gone out of fashion and everyone's a genius. Then he stumbles back into the past.
There's not really any story to this one. Leiber presents a possible future social arrangement, but without any real detail or discernable purpose.
Ted Sturgeon addressed polyamory a few months earlier in Rule of Three in the same magazine. Maybe it was a popular topic at the time?