Disclaiming Banner: I still exist. I still don't have the copyright for ATLA.
Beginnings
He didn't know he loved her at first sight. That was just weird. Romantic love came on more gradually than that. He was pretty sure it did anyways. The monks had taught them about love in a very general sense. The other kids at the Temple had been pretty young and he had been the Avatar. So, he didn't know much about that kind of love.
And really, it had been a long time since he'd seen a girl. Probably since just after his final tattoos had healed over and the traveling artist had moved on to the next temple. But at the same time, Katara had such beautiful eyes and so much passion that it made him feel a little ashamed sometimes...
After the War, they had years to figure this love thing. But it really didn't take long for Aang to figure out he liked sex.