I do think a certain amount of pressure is warranted. It IS a rare case that they truly don't know - and to deny the name when you DO know (no matter what the situation) is not on.

They do it for lots of reasons - lots of times because a friend did it and then goes and tells everyone else how they lied about it. OR they may be being threatened by the father that doesn't want to pay CS OR they are told by 'well meaning' friends that lying about it is fine.
You can't change paternity for personal convenience.