It is hinted that Ezreal is dating a Miss Crownguard. Yes That is most likely Lux and there IS an image of them together. However, Riot COULD be pulling the chains on us in this and turns things around and have it as Lux's sister/cousin (if they create a character for that even if that character is or is not playable). However, it IS safe to bet that they are a couple but as for marriage that could be a whole different issue. They are secretly dating and that could be for various reasons so unless their relationship is out of the bag, I doubt there will be any further development on that sadly.
Also not sure about the Demacian style of heirarchy... That might also be one of the reasons why as to Ez and Lux secret relationship. Lux IS from the Crownguards which are literally one of the most notable house of nobility and thus might think twice about marrying their (currently but may/may not be for long) only daughter to another not hailing from a noble line. Sure hes got am impressive title from Piltover (and Demacia did send Lux and her brother as envoys to Piltover and thus can make his status more important and their meetings more frequent) and he has many wonderful stories and is one of the champions, that might not be enough for Mr. Crownguard (Lux's father)... only time will tell.