Homo genius society
Gives off Planet Of The Apes vibes to me.
A heterosexual person from our time is somehow sent centuries into the future, to a time 3 - 4 generations after a global disaster that reduced the human population to roughly one billion. The involuntary time traveler learns that a mystery radiation during the disaster caused a mutation in the survivors. The result of this mutation is that among the Kinsey 0 (exclusively heterosexual) only the most intelligent are just barely intelligent enough to for instance learn to read and write. While the Kinsey 6 (exclusively homosexual) are the intellectual elite. Everyone else are inbetween.
The time traveler becomes a sensation, a blast from the past, being a heterosexual with an intelligence level now only seen in people who are somewhere within Kinsey 3 - 5.