you and me, can't you see that we're playing with fire, tell me now do you feel this burning desire?
The unwritten rule of War has been silently understood for five millennia, that is why no one sees the point to tell their respective leaders that they're playing with fire. Their heart breaks for the star-crossed lovers, but in their world happy endings don't exist. The cruelness of war and separate sides destroy what could have been, and the rare chance of pursing a normal life away from the chaos.
The lovers allow themselves many tryst within the short span of four years, with sweet nothings uttered and with unfulfilled dreams and wishes. But they can not keep up this charade much longer. Their respective sides would eventually find out about the affair and laugh how the renowned hero of era yearns for one of Hermes' many, many, simpleton sons. Not forgetting the fact that it happened to be the God's favourite son that had betrayed Olympus.
Grover and Ethan watch the lovers treasure their sacred moments together, both knowing it could be their last. They both come to a silent understanding.
The Fates have been cruel to Luke Castellan, son of Hermes.
The Fates have been cruel to Percy Jackson, son of Poseidon.