To be fair to him she was not a normal person but genetically enhanced and we are shown how rapidly they are able to pick up and advance in social situations. A few days/weeks for them may well have been years of normal social development for a non enhanced person. So I can sort of see how he could have so deluded himself.

But that's why there should have been hard regulations against doctor patient relationships under any circumstances. Precisely because you can't trust a self serving person who may indeed be desperate and lonely as Bashir was to impartially and rationally judge a situation like this. So you take it out of their hands. And he was rightfully horrified at his own conduct when the full realization sunk in.

That said Starfleet has some pretty wonky ideas about proper conduct for a doctor. A few years earlier everyone was on board with him wiping Kurn's memory of his life without his consent just because he was feeling suicidal. And while in Klingon society the elder brother may have the right make life and death decisions for his family, they were not in Klingon society, they were on a Federation station and he was a Federation doctor.