Congeniality
Not everyone was enraptured by the idea.
The Ocean was, as you may have gathered, a touch jealous. And old Spirits, even one as mutable as Ocean, are slow to change their habits.
Now, through the millennia, Ocean and Moon have always been close. This is true.
Their closeness often takes the form of two lovers. Granted.
Also: through the millennia there have been those who would desecrate the Balance. There have been other mortals who have given their life, and in doing so have ascended to the Spirit Realm. That is to say: change has its place, even among ones so great.
(A greater philosopher could wax rhapsodic about how the change goes both ways: over the centuries, an ascended mortal will shimmer and lose what we call humanity. But the story:)
When Sokka butted himself between the Moon and her partner, it's true that Ocean's temper got the better of him (for this century, the Ocean's form was masculine). But it only took one turn of the Moon before Ocean decided, the happiness of that young Yue had more sway than any petty rivalry.
So, Ocean and Moon had been lovers. So what? Since Yue's ascension, the Ocean had been to her as a teacher. And a teacher must let their student go, to enter into the world, and learn by making mistakes. That is the way.
...
The other Spirits and sages of the Moon's country gave their blessing, too, in time, although the Rabbit in the Moon was the last holdout. The Rabbit, who among her other duties wove dreams for mortal sleepers, insisted that Sokka get the blessing of his family before the wedding could proceed.
Katara gave her blessing at once; Aang, when asked, smiled and said yes. But Hakoda…
It was a thorny month or so for Sokka at the South Pole, for Hakoda was slow to give his blessing. Hakoda was not heartless, but surely a man should mourn his first love and then move on, not dally and pine among the spirit world.
Sokka had to prove to his father that he would honor his duties; he would keep the Southern ways alive and not fall into idle dreaming. When nothing seemed to work, Sokka let his father in on a secret dream. Sokka unfurled plans of a college, just outside of the Southern Tribe's capital, one where Tribal youngsters could learn the stories of the past, and forge the future; a school for waterbending, a school for sailing and navigating, maybe even a school for cooking.
And then Hakoda understood Sokka's vision for the future, and he saw that his son meant to live fully in the world. And that night, Hakoda gave Sokka his blessing.
And the Rabbit in the Moon said, "Alright then, what are we waiting for?"