Hehehe he painted this image in his head of prachi being the kind of girl who would only get intimate with a guy or pay attention to a guy who she is married to, an impression that he is clearly correct on, but he's weirdly disappointed thinking he was wrong about prachi just because he saw her flirting with and talking with a guy. So in a way he's admitting he finds her alluring, interesting and special compared to other girls and that he liked the things he assumed about her and is disappointed to think that the things he thought about her are perhaps not true after seeing her with sanju.
He was genuinely telling her what he truly felt about her in the precap. Ranbir to prachi " kash mujhe pata hota tum kus tarikeki ladki hon. Yeh meri bewakoofi samjho ya tumhari hushyari ki mein tumhe samajh hi nehi pai" I think he will be internally happy when prachi is proven innocent by abhi and nothing happened between her and sanju😆😆 I also like what the writers did with prachi's character. They instilled the positive qualities of Abhigya in her. Prachi has pragya's good moral values, her academic excellence, diligence coupled with abhi's high temper, zero tolerance for injustice and the habit of not listening quietly to people telling crap about her. She fought with Aliya in yesterday's episode which I appreciate. On the contrary, Rhea inherited Tanu and aliya's qualities and has no good qualities of abhi except the negative ones🤢