Spirits are neither male nor female and the Avatar, no matter their gender, can get their female partners pregnant or become pregnant themselves by their male partners.
With a smile, Aang looked once more at those he loved: Zuko, his husband, Bumi and Tenzin, his sons, Kya, his daughter, Katara and her husband Haru, their son Kuzon, Sokka and his wife Suki, Toph and her daughters Lin and Su, May, and Ty Li, before his eyes closed. Those present shed tears, Bumi, Tenzin, and Kya were mourning their mother and trying to comfort their father as Zuko shook with the force of his sobs. Soon Aang's tattoos glowed one last time before fading as the Avatar Spirit left upon his last breath.
Nine months later, a newborn girl opened her eyes as Ravaa drew the last remnants of Aang, including his memories, and locked them away deep in the girl's subconscious along with the others she had collected since her bonding with Wan, thus wiping clean the slate so this new child Wan had been reincarnated as could experience new adventures and make new memories.
"Hello," whispered the tired voice of the woman holding the babe. "I'm Senna, your mother; Tonraq, your father will get here soon, he'd better. Welcome home Korra."
* In book two of The Legend of Korra, when Korra and Ravaa were separated, the separation caused Korra to lose her past lives: all the memories, experiences, wisdom, and knowledge of past Avatars were lost as Ravaa released the souls of all past Avatars from Wan to Aang from her care allowing them to finally truly pass into the Spirit World. With the new bonding of Korra and Ravaa, instead of Wan, the new Earth Avatar will be the reincarnation of Korra.