When the two were alone in jai's room, roshani told jai that only two of them knew about their broke up. That raaz lies between them only. If it unfold its not only affect us but also his wife bani and altogether the whole family. Jai was very aftraid on hearing this.
When doctor asked roshani whether she met with an accident in the past, both jai and roshani stunned. She said that she met with a brutal accident in the past.
So my presumption is this. Jai and roshani met with an accident in the past and due to that accident jai is unable to become a father due to medical reason. I think this is the reason which also affect bani becoz she is jai's wife and which gives feeing of pain and horror to jai. If the reason would be reverse, means due to accident roshani was unable to become a mother, then how bani will be affected? Bani has nothing to do with that as now there is no relation between jai and roshani.
2nd possible reason is jai and roshani have a illegitimate child. But looking to the bani who is very understanding, mature, having a scrifice nature and broadminded, if jai have a child, I don't think so bani will be terribly upset (as per roshani's thinking) or not ready to accept the child on knowing the fact. She knows roshani is jai's ex-flame. Ya she will be hurt if after committed their love for each other, after considering each other as a true better half, jai would not have told her about his past with roshani. But that stage is little bit far at this moment. But again looking to the jai's nature, he will tell everything about roshani and so called raaz to bani after committing their love for each other. He will not hide anything from bani.
i think 1st reason is the so called horrible raaz.
love
kiah