honestly at this point if you don't think there was something genuine between sally and poseidon idk what to tell you because it's so clearly there. like i know the queen among woman quote is probably overused but i need you to understand these two sections in conjunction with each other.
percy goes to ask poseidon if he wants to come visit sally and poseidon visibly gets upset about it because he cant (personally i think that he can't because sally has specifically told him not to in the past (which is supported by the fact that percy has memories of poseidon visiting him...which likely would have continued, at least in secret, unless sally had told poseidon not to do so, in which case i think that poseidon would have listened to her because he loved her).
poseidon being visibly upset coupled with the fact that he's talking about what him and sally had wistfuly--like come on. he obviously misses her and we know she misses him just as much from the montauk scene in the beginning of the book. l
ike it's just so fucking obvious to me that this was more than just a summer fling. like this wasn't a god seeking out a casual hookup, this was a god that fell in love with a mortal and is feeling emotional repercussions from them not being able to be together. the effect that a mortal has to have on a god for them to be upset over the loss? considering the gods have been around for millennia and experienced losses over and over again?