I think Kunal's reaction right now is a combination of different things:
1. His body is still trying to cope with the drugs that stupid Richa put into him. He's still messed up and can't remember exactly how everything happened (I hope this doesn't mean he forgot Richa made a move on him 😭 ). Women who are given date rape drugs like these often suffer from memory loss and that's actually sometimes worse and more scary than remembering the incident because you don't know exactly what happened to your body.
For someone like Kunal, this kind of loss of control must be devastating. I think he really is afraid he forced Siddhi to accept him in bed after seeing the broken bangles, and she's putting on a brave face for the family's sake.
They went from shouting to kissing and missed the whole talking part in between. 😆 So in his mind, he went from being best platonic friends with Siddhi to sleeping with her in the blink of an eye.
And that too after he evidently drank too much at the party - enough to cause a blackout (he assumes) - and definitely enough to apparently make sure the consummation was a little rough. I mean bloodletting on your suhaag raat is not something most men aim for, esp honorable types like Kunal. It's clear he went at least a little caveman on poor Siddhi (who seems to have no complaints ☺️ if only he was in any condition to notice).
2. He confessed his love for Siddhi but it's a love that he doesn't dare confess to even himself when he's sober. Even when he was high and confessing his love to Siddhi (aka Richa) he kept begging her not to be angry with him.
Siddhi for him is basically Mother Teresa. She sacrificed her whole life for the sake of his brother's love and even found a way to take care of him, Kunal, despite hating him. You don't have those kinds of feelings for Mother Teresa 😲
But the point is, Siddhi is a real woman, not some saint. He really has NO CLUE that Siddhi's flirty bits of conversation and the way she takes care of him actually mean something more, er, physical.
And how could he when his one experience with love is Richa who only ever thinks about herself. He's the one who was always taking care of Richa, cooking for her, doing things for her, going to fight for her. He can't relate to Siddhi being his equal in love even though Anand pretty much spelled it out to him because Richa is his role model for a woman in love. 🤢 He doesn't know Richa is actually just a role model for psychos.
Today, when he was trying to make sense of everything, he couldn't complete the thought of his being in love with Siddhi even in his own mind. He completely shied away from it.
3. His guilt that he slept with Siddhi on Anand's barsi. C'mon yaar! You wake up disoriented from what appears to have been a really hot first time (you go, Siddhi! 😳) and find out it's your brother / her ex's barsi? Major mood killer. 😆
4. His conviction that he's not able to keep Siddhi happy in spite of everything he's done to change himself. I have no idea why he feels this way but that was the point of that scene in which he goes back to his old hangouts to feel sorry for himself, I guess.
Now we know that this is all a load of crap and Siddhi can tell him the same if they ever talk but the moment poor Siddhi is informed of Kunal's mahaan decision, she's going to think Kunal is divorcing her for Richa. 😕
As far as she knows, Kunal told Richa he loves her, broke up her engagement, came home to find Siddhi in one of her moods, tried to make her stop, things really PROGRESSED 😆☺️ and then the next day onwards he can't look her in the face, screams in the bathroom, and goes away to Pune so he can come back and announce their divorce. I bet she feels really cheap and betrayed
So Kunal is a acting like a total dumbo but imo, once he's over the trauma, he'll be better.
Or maybe I'm just thinking too much about an Ekta Mai ka soap. 🤣 This is probably more analysis than the writers have done! 😆