Because it is not (i'm not good at sarcasm).
Sex it's like other concept or ideas, it's another s tool of device for exploring certain theme and plot of a fiction. While sex is usually done and seriously being talked about by adults, it doesn't mean that containing the element of sex itself makes a fiction turned mature. When its handled poorly, sex will make the fiction actually looks more juvenile than mature, like teenager having sex just to prove themselves how mature they are without thinking of the consequences.
Well, for Moral Panic, don't get me wrong. I'm not against fiction being exploring sex as theme of relationship. Catherine show how great it is when done correctly. But saying that human relationship is built hugely on sex is pretty.....narrow? There are many relationships that not built around having sex. Sex is just another facet of human relationships and its certainly by no mean a huge part of it.