The thinking that produces this stuff IMO comes out of many years of a particular way of thinking, that's sort of creeped in.
The idea that kids will do sexual things anyway, so it's more protective to talk about them.
The idea that young people can and should decide when they are ready for sex, and it's just fine and normal for anyone 16(ish) or above to do so, have any number of partners, etc, as long as they are safe. This is a little different than saying, lots of kids will do it even though we adults know its usually best to wait longer, and so the emphasis on giving advice changes.
Flipping Masters and Johnson. It's become a truism that whatever sex acts are popular fads now, they always have been even if people used to be secretive and ashamed. As a result, people didn't think - gee, maybe it would be better if anal sex wasn't considered just like regular sex and normalised". People also fail to realise that you can't treat it totally individualistically - people's expectations are set by what they see and hear in the wider culture.
Inappropriate understanding of child development generally. The same people who think that it's a good idea to talk about genocide with six year olds may think that talking about adult sex topics is a good idea too. Because they don't understand children't mental development.