Heaven's eyes widened in surprise. She was quiet for a long moment, then her eyes darted nervously around. Zamiel knew she had a lot to consider before getting married, and that is why he proposed. It was the perfect way to solve the problem.
He didn't want to mark her only. What would that mean without getting married? What role would he have in her life? She lived in the human world, and the mark would only tell demons she was his.
If she was going to be his, he wanted the whole world to know. Demons, humans and witches. He didn't want confusion from his or her side. He wanted her to be his in everyway. Every part of her, human, demon or witch, he wanted to have it all. If a part of her wasn't ready or didn't want him, then it would be better to not mark her again.
Heaven seemed confused. He knew she would be. She had her dreams to become a ruler, and she was still young with a long life ahead of her. He didn't expect an immediate answer.