Your wife cheated on you, and you have been raising some SOBs kid. You find out the truth. Do you honestly want to torture yourself every time you see that kid? The kid isn't exactly going to be a font of good memories, that kid is a reminder of his wife, and some guy she slept with.
You don't have children do you SOMEKID? You've never loved or been loved by a child, have you? I can hardly believe you have based on your responses here.
If you did you'd know that loving a child is just that - loving a child. Teaching them, playing with them, being with them, helping them and all the rest - good and bad - that goes with being a parent.
Finding out you weren't the sperm donor doesn't make all that go away. It doesn't take away the good times, the bad times, the years of experiences simply because you weren't involved during the 10 minutes it took to conceive the child. You don't punish a child for the sins of the mother.
Donating the sperm takes 10 minutes. Being a father takes years and years. You don't throw away the years just because you didn't get to enjoy the 10 minutes.
Keep this in mind KID - there's an old saying that goes like this.
Momma's Baby - Papa's maybe. It still holds true unless one is so insecure in their manhood that they feel they must go the DNA route.
I'll say this one more time and I'll say no more in this rather disconcerting thread:
You don't punish a child for the sins of the mother.