There aren't any
good choices in this scenario at all. When faced with such a dilemma, I believe that one must consider the solution that will bring the greatest good. YMMV.
For me, abortion would be out. Although I support the right of everyone to choose, I could not end a life because of a mistake to which I was a party.
Adoption is more palatable, but how could I send this child to an unknown fate when I know that we could provide a good, happy home for them?
There's only one choice that I could make, to raise the child as my own, maybe revealing the truth to them when they are adult enough to know. After all, he or she is half my wife too, and furthermore is a fellow human being that deserves no less than the love and respect I give my own children.
Would it be hard? Would it disrupt our lives? Would it change the plans that we have made for our later years? Of course it would, but that changes nothing. It's still the right thing, in my mind, to do.
-B