Arjun and Purvi just got an opportunity to start fresh. Not many people get that in life. Yet, these two can't maintain honesty.
Arjun should have told Purvi about sleeping with Ovi again after it was her telling him to get back with Ovi and Arjun/Ovi were indeed married. So if the baby is Arjun's then it wouldn't be completely shocking news for them.
But more than anything, I blame the crappy writing and Ekta's genius ideas. I want Ovi out of Arjun/Purvi orbit. These "amar premis" are perfectly capable of creating tension and unnecessary drama in their marriage just by themselves. Please keep Ovi out of this mess and write more creatively for her!