From my understanding, Ranbir gets shit on, in terms of his public image when it comes to relationships, because he's known to be a cheater. Not only did DP and Sonam call him out on it in KWK, he himself admitted as much. Not to mention, there's a lot of publicity surrounding his cheating issue and DP-RK still milk the relationship when it comes to promotions for movies they do together. No matter how the relationship was, at the end of the day, it's been pmuch established that DP was madly in love with him, he cheated on her, she fell into depression. The entire narrative points to DP as the naive victim and RK as the horrible cheater.
Regarding DP's relationships, she seemingly bounces from relationship to relationship, but there's nothing really scandalous about them, to the public eye. She hasn't really been singled out as a cheater or anything. Of course rumors about her relationships do exist, but I doubt they're as damning because they're just rumours, she's never even acknowledged them or confirmed them. But, she does get shit on when it comes to her relationship with RS b/c she's seen as being too cold or non-committal to RS's public adoration of her. But again, it doesn't really hit the level of cheating.
TL;DR -- RK screwed himself over by admitting to cheating and DP has excellently used the victim narrative/ PR/ radio silence in order to secure her reputation
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