And I'm not saying this as a relationship, just their family-ship i guess.
Anyway, what do you think would have happened if Annabeth ran away with Luke when he asked her?
I personally think that Annabeth would have died. Kronos wouldn't have given up Luke as a host so easily so he would have found a way to kill Annabeth and blame the Gods and Luke would have easily destroyed Olympus as Kronos (well really Kronos would have as Luke but you know what I mean)