Instinct
Princess Ellen was surprised by this question. She did not know why he was suddenly asking if she wants to be his queen. So for a short moment, she just stared at him without any reaction.
"It is not up to me, your highness," when she finally realized why he might be asking such a question.
"What if it was up to you?" Prince Sebastian asked. He was trying to see through her thought. Although he had sounded very confident when he spoke to his mother earlier he still had little doubt.
Princess Ellen looked at him. "What if I say I was to be the queen that seat next to you, will that stop you from marrying another now or in the future?" Princess Ellen asked.
She has been doing some thinking and she realizes that she never had a guaranty that he will not pick up another wife. So what was the difference if he did it now or later? Even if she is the queen that sat next to him, it will not change the fact that he could pick up another wife or a mistress.