It seems there are people who are confused about the gay kiss in Shinsekai Yori and fail to understand why it occurred on grounds of there being "no build up to it" or due to it "coming out of nowhere". This post aims to dispel such ideas by explaining why that event happened.
So, it was established previously in the show that Bonobo DNA had been spliced into the humans of that era, making it so that they engaged in sexual intercourse whenever they became angry. This was used as a method to subside that anger and to ensure peace in the telekinetic humans' society. This relates to the show in that Shun and Satoru's copulation is caused by this mechanism.
You see, Shun was on the verge of becoming a Karma Demon (a monster that a telekinetic human turns into when he becomes too powerful and loses control of his emotions) when the kiss and the following sex took place. The sex acted as a means for him to relieve himself emotionally, thereby gaining greater control of his powers and averting his transformation. However, as we see later on, it didn't work and Shun was nonetheless forced to leave society.
As for why he didn't engage in sex with a female, the society has laws against heterosexual sex for teenagers (better specified in the novel) that were put in place to stop its members from prematurely becoming parents.
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