Shin Sekai Yori

In the anime all of their classmates(and probable the whole school) are in same sex relationships(expect Mamoru) they most likely used the idea of sex friends as a mean to stop violent behavior using orgasm, similar how Death Feedback is used to stop them from killing.

Its possible that in the future they will use the suggestion to create opposite sex partners for breeding, since Bonobos have sex with anyone it wouldn't create a suitable environment for child raising, so creating actual man and woman couples wouldn't only create a chance for breeding but also another feedback, and the breeding could create powerful Cantus users like Shun.

Its also very likely that what they feel towards each other isn't necessary love, but rather an "idea" of love, so they can let it go when suggestion is applied, which is why people seems to forget about children's that disappears(there seems to be exceptions like the love of a parent to a child, nor the effects are strong in certain individuals like Saki, don't know why).

But I still don't see a reason for the explicit yuri scene.

Bonobos are known for having sex with anyone, including same-sex partners; even in heterosexual males and females. If a society and genetics had been modelled after them, then the same would probably be true in that human culture and probably not frowned on or even really commented on.

For example, in the anime, the children are told by the walking glowing library that the reason that they feel sexual attraction to each other is because the new societies of the world modelled human interaction after the Bonobo monkey. Then, the anime tries to force a scene of sexual attraction into the chase scene with the Spider clan. Too forced. The manga, on the other hand, (while it hasn't explained the above attraction scenarios) does, from the get-go, have the characters engaged in sex-like acts with each other. Which, if anyone knows anything of the Bonobos, is a daily thing.

the children are basically brainwashed by the society to keep them from remembering those classmates or friends or whatever that have been removed from society because they've been deemed unfit. The time-skip is appropriate; just imagine if you just went through something, then were made to forget it, it would feel (for a split second) like there was a blank in your memory; then the conditioning would snap in and you would completely forget about what they wanted you to forget about. At least, that's what the manga is implying

I mean Satoru x Shun for real? how did it end like that? (-.-) I don't really mind if someone can spoiler for me...

BTW Shun is fucking hot