Sokka and Katara were raised together and sharing all things, be it the tent they slept in or the boat to travel. They are also close because they’re cold in their homeland. Sokka and Katara are prepared to use any means to keep warm. Fucking is not only the most efficient option, but it is also the most pleasant.
Avatar Aang must master the art of learning to manage many things, and his sexual excitement is definitely one of them. Also, it is evident that he may need assistance with this and who could assist him more than his more sexy friend Katara? If he fails to master the trick on the first try, he’ll still have fun with Katara.