Babies and children are innocent. Whether they're borne out of
wedlock, or in an illicit and adulterous affair, the babies / kids
remain innocent.
If I were a husband and my wife would come up to me to say that the
child that I've thought of as mine all along turns out to be
another man's, due to an adulterous affair she had after marriage,
I would hate the woman for her infidelity and for cuckolding me,
(sorry, pretending to be a guy with a manly ego here!
), but it would be
hard for me to hate the child that I've brought up and loved all
these years as my very own. Afterall, the child did not ask to be
conceived in this sinful manner so it's really not his/her fault.
Yes, I will continue loving the child because it is possible to
love children who are not your own flesh and blood as your own, but
I do not think I can continue living under the same roof with the
unfaithful woman. 
Now, for the reverse situation... if I were the wife and my husband
came up to reveal that he has a child, it would depend... was it
before or after marriage. If it was before marriage, I'd ask why he
had to withold the truth till now. Such issues should rightfully be
discussed after marriage. I'll definitely smart from the shock and
hurt, but I suppose in time to come, I'll accept that and will want
to get to know the child and slowly play a more active part in
his/her life, depending on how his/her mom feels about it.
If it was a child conceived whilst being married to me, woe to
him!!
All trust is broken
and there really is no excuse for cheating behind my back and I
have zero tolerance for infidelity. I will pack up and leave, and
make a call to the lawyers to prepare the papers. 
I've always thought that after marriage, I'd either remain
childless, or adopt a child. It is possible to love a baby / child
who is not your own.