Sorry you are so upset YLL, but I totally see Petfinder's point on this one. I'm sure they have "breeders" attempting to use their service all the time, when they are committed to helping homeless animals and not breeders or irresponsible people who let pregnancies happen accidentally. I understand none of this applies to you, but Petfinder did not.
Stating 'father unknown' would certainly make you sound like a very irresponsible breeder (or at least irresponsible pet owner) if one doesn't know that purchased her already pregnant. Just try to see it from their viewpoint.