"You're an idiot," Narcissa said, turning to face him finally. "You want to turn your whore into a martyr? That's what will happen, Lucius, if we use your oh so clever solution. The Order just loves their martyrs." The people currently in power didn't believe that Lucius had ever been under Imperius. They couldn't prove anything, of course, but public opinion was not on their side.
Of course, she was not unaware that part of the story would hold up. No one would be surprised or blame Lucius for straying. He was a man, it was expected, he was under Imperius. The child wouldn't be blamed of course, and all sympathy would be for him. Even the whore wouldn't be held accountable; Dumbledore's people would see to that. Cry rape and torture, and all of that messy business. No, the ones who would be hurt by this would be her as the blind wife who stayed with the man who strayed, and her son. The press would make this her fault if she didn't get out in front of it.
"You realize that in every scenario, I'm the one who is going to bear the brunt of the blame, and be raked over the public coals? Whatever direction we go, acceptance or denial, I'll be the one scrutinized for it." So the answer was to accept it, make herself into the hero of the story. As much as it absolutely killed her, she was going to have to be the one to welcome the damn brat into the family. All without lying, because Lucius was right about that, not that she'd give him any credit for original thinking.