He does. He does know her better. He knows that she would never turn her back. But, he has to offer her the out. He has to offer her the chance to not be caught in this web of politics that has surrounded the pair of them now.

But he's still Zuko at times. "I could marry Toph." Toph would kill him. He knows that too, but she's nobility from the Earth Kingdom, so that does carry some weight amongst political circles.

But he's not being serious. His advisors would go for it, but the girl herself would lock him in an earth prison with his hands captured and pummel him with rocks. If she was feeling nice and didn't just cackle her way out of town.