Spoilers: Once again contains spoilers for Season 2, based on the episode 'The Once and Future Queen'.
Truth
Throughout his explanation to his father, he cannot keep his eyes from drifting to her. The memory of her smile, her lips on his is something he does not wish to forget. But, at the same moment, he knows it cannot be.
There are many explanations he could give. He could tell her it was simply a spur of the moment choice, that in the end in means nothing. He could tell her he is sorry.
He tells her that his father would never allow it.
When he sees her melancholy smile, he is glad he told her the truth.
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A/N:
I was glad that Arthur told her the truth rather than simply brush off the kiss as nothing. Shows a maturity, I think. Once again, exactly 100 words.