In Shin Sekai Yori, there are kiss scenes between two men (Shun and Satoru, I think).
Are that scenes also described in the original novel? Why does it have these scenes since it is unusual for typical anime, and it isn't in the yaoi or shounen ai genres?
This behavior is straight from the novel and is described by the Minoshiro in episode 4. The answer will be revealed through the course of the story, but a quick witted watcher and/or reader will have his/her answer at episode 5.
As the Minoshiro explains, necessity dictates that the human society on the brink of destruction must be reformed into a society of love, like the bonobos (http://en.wikipedia.org/wiki/Bonobo). It is assumed that the grafting of bonobo DNA into the human genome causes certain attributes to be transferred. One of the explained behaviors is the relieving of stress through sexual activities (as observed in episode 5, when captured, Saki and Satoru engage in such activities to reduce their axiety and fear).
Combining this information with real-world information can then be extrapolated as:
One of the other observed behaviors in the bonobo, is the bi-sexual nature, which is present in apes (http://en.wikipedia.org/wiki/Homosexual_behavior_in_animals#Bonobo_and_other_apes). To form a society of love as the intention was, it can be assumed that the effect is already present in the human genome and may have been strengthened by the DNA grafting. Saki and Maria have a similar relationship.
So, why did they include these scenes?
That's the background of the humans and thus a critical part of the story.
I'm not sure about if the relationship between Satoru and Shun is in the original source material, but an anime/manga does not necessarily have to be a yaoi/shounen ai for it to have a gay couple in it. Yaoi and shonen ai have the relationship at the core of the story, making it a romance anime/manga, while Shinsekai Yori has this relationship in it to supplement the plot, rather than being the focus of the entire story.
