Moral dilemma - your opinion appreciated

Hi,

My wife and I have a 9 month old child, which from the time my wife found out
she was pregnant, we both knew I was not the father. My wife claimed from day
one that she was forced upon by two girlfriends she was staying on holidays
with, who had just transferred their attention from some guys and must have
somehow transferred semen. Of course I never believed this, however couldn't
100% dismiss the possibility either.

So I had the option of abortion, divorce or adopting the child as my own. I
chose the latter as our marriage was strong (at least on the surface and love is
blind). Now when the child was 9 months, I suggested a timeout from our marriage
for a breather and to show how serious I was about stress I have from my
overdemanding and financially ignorant wife not giving me time to earn money (I
work from home). Upon hearing this, she admits to having a one-night-stand with
a guy overseas and immediately calls a friend of a friend of his to pass the
news on that she has his child. She travels half way across the world to meet
the real father after 2 weeks of separation, tells me that they are both madly
in love after another 2 weeks, and brings him back to live in our house after
another 2 weeks (I had to leave the house because of depression, memories etc).

Now financial settlement time, my wife demands a 60-40% breakup, with the extra
10% because of the child (which is about US $70,000). She has earnt as much
money, if not more, than I in the relationship.

Do you think morally I have a financial responsibility for the child, given that
he wasn't mine (yet I 100% accepted him) and that my wife is now in a
relationship with his real father (who has quit his job and moved half way
across the world)? I can see it both ways.

Your opinion appreciated.

Simon