What I don't like is that Aang falls in love, especially since monks are suppose to get rid of bad qualities within them, or something like that.
You say it like love is inherently sinful.

I know he was supposed to let go of it to unlock the Avatar State and he is stated to be a monk, but on the other hand, celibacy can be hard to commit too... AND he's the last of his kind. Meaning, if he hadn't hooked up with Katara and weren't to have Airbending children, then the Avatar Cycle would be stopped in the future after Aang himself dies. Ultimately, it'd be for the best in the event that history were to repeat itself in that universe.