It’s probably because he’s seen how easily she accepted Kuki as her own and showered her with so much love. When you see someone do that for your own child without them ever using it against you it comes across as a much bigger thing for the parent. He’s also seen how much she loves her close ones, even the ones who don’t deserve it. Bajaj himself has been deprived of love full stop. So Prerna naturally attracts him.
Agree with your points.but I have a doubt. Seeing RB it seems he never expected all this whatever little she did so far.
So just like us who believe in the theory not to expect anything from this show, even though he married her, he never expected she will do all this whole heartedly towards kuki.