As I said before, I had a lot of shipwreck/island books to compare Nation to when I was reading it. But the other piece of literature I kept bouncing it off was Philip Pullman's His Dark Materials trilogy. The parallels are pretty obvious - boy and girl from different worlds come together, have comic misunderstandings, join forces to do good things; there is anger at gods, there is lots of science, and though true love is found they choose to part for the Greater Good. Pratchett is far nicer to his gods (I'm not sure they deserve it) than Pullman is to his, certainly.
But it's those two endings that interest me. Pullman gives us teenagers falling in love and having sex - something which horrified a number of people I discussed the book with, who then decided to interpret it as them not having sex. No, I don't know how either. Pratchett does not give us sex (though it might have happened), but he does give us sexual desire - at one point Daphne is just looking at Mau's body and the way the muscles shift under his skin and so on, and in some way this is an even bigger deal than that bit in The Amber Spyglass - because it's not written as being all about love, and it's such a relief to have a teenaged girl character want someone.
And yet both books, having suggested that sex (or the desire for it) is a normal part of growing up, then end by parting their characters by sending them to different continents/universes and effectively ensuring that they cannot have any more sex. Which is, I suppose, preferable as an ending to the Deathly Hallows sort where everyone ends up living happily ever after for the rest of their lives with their first partner. But still, I wonder.