"He cheated, Alec. He lied and he thought he loved someone more than your mother. Enough to leave her. That is not you."

[His voice breaks on the last part because he believes it with his whole heart. Just like he believes he might be doing something wrong.]

"You love Magnus. You don't lie. You're not cheating. You've been honest and open and...and as far as I know Magnus isn't pregnant, so..."

[Even with the teasing his emotions are at war. He wants Alec to be in his life. But what he wants may not be the best for Alec if he thinks he's just like his father for having a heart big enough to care about more than one person.]