Munna loves Munni. Period. He didn't know that Munni was pregnant when he pushed her off the Bomb and stood on it. He didn't know Munni was pregnant when he said "Main Bani ko khona nahi chahta". Yes he is not able to admit either to himself or to Munni just that.
Face it. If an heir was so important, Roshni or Sonali would have readily obliged him. So would many other women, just for his money. In fact on the C night, Jai could have chosen Roshni rather than go back to an uncertain future with Munni.
Yes, the baby is important but only because it is both his and Munnis.
Rgds
Priyanka