it depends on the circumstances, if he had the child before I embarked on the relationship I would be okay about it but upset that he couldnt be honest and confide in me about it.
If he had the child as the result of an affair (within the relationship) then that would be a different story entirely...
My aunt had a secret daughter who her parents adopted and brought up as her sister, back when it happened though I'm guessing that wasn't uncommon.
I think my uncle knew but it only came out generally a long way down the line, well after my aunt left my uncle (after 31 years and 3 more children) and went back to her first daughter's father and they are still very much together now.