So unless Elma x Tooru is gonna be canon (I wish it would, for the record), I don't really see the big problem with her getting married. Like, she's not doing anything of importance in the human world anyway, and of all the dragons she's the only one who lives alone, as she doesn't have any human of significance in her life. If the fiancé turns out to be evil that would be one thing, but assuming he's not I don't think this situation sounds all that bad. Plus, I'm kinda tired of seeing all the dragons fall in love with humans anyway, it'd be nice to see at least one relationship between dragons.