Disclaiming Banner: I make no claim to ATLA. I am waaaay to poor for that.
Heir
As a young man, he had not wanted children. He understood as Crown Prince, he would need an heir, but child rearing was never a task he looked forward to. His brother Ozai was still a toddler when Iroh had married and encounters with the sheer unpleasantness of infants left him hesitant to sire one of his own.
Of course, there would be servants to take on the bulk of the labors. Surely the Dragon of the West wouldn't be expected to oversee both the front-lines and whatever brood he and his wife had.
Two years into his marriage saw the birth of his son and heir. It was for the best. He and his wife were both young and this auspicious start to their married life would secure the future of the nation. Iroh would go back to the front-lines and capture more territory for the Firelord. He would visit "home" twice a year and plan to sire another child, just in case.
He never gave it much more thought until Lu Ten began to speak and didn't know Iroh was his Father.