What can happen in a world with sperm donors

From an advice colum entitled My Wife Is My Sister:

When my wife and I met in college, the attraction was immediate, and we quickly became inseparable. ... She had sought out her biological father as soon as she turned 18, as the sperm bank her parents used allowed contact once the children were 18 if both parties consented. I never was interested in learning about that for myself, but she felt we were cheating our future children by not learning everything we could about my past, too. Well, our anniversary is coming up and I decided to go ahead and, as a present to my wife, see if my biological father was interested in contact as well. He was, and even though our parents had used different sperm banks, it appears so did our father, as he is the same person.

I think I've said this before - the biological parents belong on that birth certificate. Anything else is absurd. (And there's the question of whether or not paternity testing should be mandatory to consider as well - genes matter).