Ha, absolutely not.
If I was engaged to be married to a man, I would think that I would know him well enough to know he has children. In which case, a pregnancy would be no big surprise or shock. And, it is a known fact that when one of your children has a baby, you become a grandparent. No shock there either.
Now, how that could be a deal breaker is beyond me. What is the problem here, I do not know. Or, maybe he decided to leave his estate to his grandchildren and not his wife, perhaps? I just cannot figure out why it would be a deal breaker if your fiancé's child decided to have a child. How on earth does that effect your daily married life? Do tell, please, do tell.