I think this is the point where we need to make a legacy like chalange. The chalange is to have as many single sims about the naborhood with very distinctive genetic traits like huge ears or dark blue skin and see how many end up with babies after a certain ammount of time. Check the genetics and see if the very distinctive trait has carried on to the random spawn babies that only have one parant.

I think its the only way to see if its some sort of odd bug.

I don't think it's a bug at all. I've had several sims who have had babies while I'm playing another family. Sometimes both parents are listed, other times only one parent is. I think it's a nice feature. Not every child in RL knows who their mother/father is, why should sims?

Chances are they either don't want the parent to be revealed or the sim has since moved out of town.

i get that this is a glitch and I get that it is fustrating but I think it is so freaking funny that the Sims 3 made Jesus's return a host of immaculate conceptions. Come on...
I'm not God and I play the Sims to control them not give them free will.
Oh, and the Jerry Springer comment was hilarious.

I also have a theroie that single sims with sudden kids may not move into another lot when the relationship bar is at full and all should be going well.