Would you father someone else's baby (details inside)

According to some researches 1 in every 25 fathers are (unknowingly) fathering a child that isn’t biologically their own (other studies say two percent). sources:1 and 2
So, men, suppose you would know that the coming baby isn’t yours, but somebody else’s (your wife/girlfriend confessed to you that she had a fling and, one time, slept with another man and got pregnant. She tells you it was a one time event, and she has no loving feelings for the guy.)
Is it better to know beforehand that the child isn’t yours, as opposed to hearing/finding out later in life, when you raised and loved the child for years already?
What would you do? Father the baby as if it was your own?
Ask your wife/girlfriend to go for abortion?
Leave her?
And ladies, what would you do, were you in a situation like this?
Confess it, or keep it for yourself and let your boyfriend/husband believe that he is the one that got you pregnant?
I realize it is (can be) a tough one, and a pretty private thing that you might not want to share with us.
If you want, both boys and girls…., thanks!