Take into consideration how Aang might feel about this. Back in our time they are together, and he is aware they eventually have children and grandchildren. Even if they aren't together now, there is still a lot of history between the two of them.
Part of why I am suggesting you try a few casual dates before that point. See how you feel after meeting a few different people. If you still feel the same, you can then have that talk.
I might phrase it a little differently. Besides which, how do you know Katara feels the same?
I will entitle this thread "Iroh teaching Zuko the Bro code"
Take into consideration how Aang might feel about this. Back in our time they are together, and he is aware they eventually have children and grandchildren. Even if they aren't together now, there is still a lot of history between the two of them.
Love can cause great rifts between friends.
Ha ha XD
I know all of that.
But how am I supposed to... do that? Hey Aang, I want to date your girlfriend from back home, is that okay?
[He slumps.]
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I might phrase it a little differently. Besides which, how do you know Katara feels the same?
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I don't.
[He hasn't gotten that far, yet.]
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[He strokes his beard]
You can spend a little friend time with her and try to gauge her feelings, if she gives a few signals then you can discuss with Aang.
[He finishes smoothing out the table]
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Looking a bit dejected, Zuko rises from his chair and heads for the door.]
Thank you, Uncle.
[Though he doesn't sound particularly enthusiastic, more like discouraged.]
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Before you go, what does a teapot wear to bed?
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I don't know.
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A nightea!
[He grins broadly]
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Good night, Uncle.
[And with that, Zuko leaves.]