When the ship docked in Republic City, Zuko tried to keep his distance from Katara and from him. But avoiding Katara altogether was impossible since there was a wedding to prepare for, and she was the bride, and he was the best man.
Zuko only spoke to Katara when the necessary pleasantries needed to be made. A bow, a "My Lady," and then an excuse to move on. The sad look in Katara's eyes whenever Zuko rebuffed her was enough to break his heart, but she seemed to understand his reasons.
"You could stand to be a little friendlier," Aang whispered to Zuko during a banquet one evening after Zuko went in the other direction when Katara approached.
On the wedding day, Zuko found a scroll on his bedside table. The paper was warped, and the calligraphy was blurry in spots, obscuring a word or two here and there. Whoever wrote it must have been in tears.
"My dear Zuko," it read. "You were right. Aang is a good man..."
Aang had instantly developed a puppyish devotion to his bride. He followed her around, always ready to do what might please her and fetch the types of treats and trinkets girls loved: Tarts and pastries snatched from the kitchen; knickknacks and costume jewelry sold by vendors in one of Republic City's many markets; fruits and berries picked from the garden; seashells and wildflowers gathered during their trips to the city's parks and beaches.
So Katara wouldn't get homesick, Aang had the cooks make her favorite Water Tribe dishes, even though most of them contained meat. The few Aang didn't have a moral objection to eating, he swallowed with poorly hidden disgust. They also visited the Water Tribe cultural center, where Aang spent the entire time stifling a yawn and wincing at having to handle objects made from fur, animal skins, and bone.  Zuko had watched with an indulgent smile. You couldn't say Aang wasn't trying.
Zuko continued reading the letter, knowing that "Aang is a good man" had to be followed with a "but...."
"I'll try to be happy and love him as he loves me but he'll never be more to me than you are. There won't come a day that I don't wish you were in his place."
Zuko rolled up the scroll. Reading more of this would be too much for him.
Katara liked Aang well-enough. They had a lot in common, such as an interest in calligraphy and painting. Zuko often came across them sharing stories about grumpy old Master Pakku, who they'd both studied water-bending under or using the fountains in the garden as a sparing field. But anyone not blinded by Aang's twitter-pated naïveté could see that Katara didn't feel the same way he did.
Whenever Aang tried to tell her he loved her, Katara avoided looking him in the eyes and did her best to change the subject. An uneasy brush colored her cheeks when he kissed her. Zuko could understand how she felt. Katara didn't have the heart to break Aang's.
Telling Aang that she didn't love him would be like telling a child that the spirits didn't exist.
Curiosity to know what else Katara had to say drove Zuko to open the scroll again and read more.
"I will never truly belong to Aang. You are my true husband after what happened between us on the ship. But, we all have to play our rolls in this farce that fate has created." 

Katara's smile throughout the wedding could have been painted on the mask worn by an actress playing the role of a joyful bride. The mask didn't slip once the entire evening.
She certainly looked the part in her white bridal hanfu, embroidered with lotus flowers and a matching cape trimmed with fur.

Zuko couldn't take his eyes off her.
The best man staring at the bride would certainly raise a few eyebrows, but Zuko didn't care. He was allowed to be selfish in this one respect.
Zuko toasted the bride and groom's health, happiness, and prosperity and agreed with everyone who said the strapping young Avatar, with his endearingly childlike grey eyes and winning smile and the mysterious Water Tribe beauty made a handsome couple. But, he couldn't help but imagine how things would be different if he had done what he should have: told Aang that Katara was his. As the world's most eligible bachelor, Aang could have any other woman he wanted but not Katara.
"Looks like I'm not the only one who's miserable." Meng, Katara's handmaid approached the table where Zuko sat, sipping his umpteenth glass of plum wine. She carried a plate of egg custard tarts which she placed in front of Zuko. "If you're going to drink, at least eat something."

Zuko took a bite of one of the tarts. "Thank you, Meng," he said after he'd chewed and swallowed.
Meng's eyes looked heavy, and her mouth quivered as if she were trying not to cry. Zuko's heart went out to the poor girl. It was obvious that she was wishing herself in Katara's place.
Wherever Aang went, Meng's adoring gaze followed him. She blushed and giggled when he was spoken of or when he condescended to show her any attention.
Zuko finished his current glass of wine and poured another. Meng was another stain on his conscience. If he'd spoken up, Aang's illusions would be dashed, but he might eventually find truer happiness with this sweet, pretty girl who actually loved him. But it was too late.
The ceremony had been performed, and the marriage certificate signed. Aang and Katara were husband and wife.
Meng knelt down on a cushion at Zuko's table. She grabbed a tart and daintily nibbled at it.
"How is Katara going to..." Zuko furrowed his brow. How should he put this without being too crude? "...Pass the inspection?"
A smirk from Meng told Zuko that she understood his meaning. Aside from the Zuko and Katara themselves, Meng was the only person who knew what had happened between them that afternoon the day before their ship docked in Republic City. A handmaid was always privy to her mistress's secrets.
"Oh," Meng said. "A vial of blood hidden under the pillow. She'll sprinkle some on the sheets after Aang falls asleep."
Zuko took a sip of wine. "I doubt Aang can tell the difference." At least there would be one bumbling virgin this wedding night.
"That's nasty of you to say, especially for someone who deflowered his best friend's fiancée."
"Agni forgive me." Zuko would never forgive himself.
Meng leaned closer to Zuko and, in a conspiratorial tone, said, "I watched while they signed the marriage certificate."
"And?" Zuko replied. What did he care about marriage certificates?
"When Katara signed her name, there was an ink blot." Meng lowered her voice, taking more care that they not be overheard. "Where I come from, that's considered a bad omen."

Zuko nodded. In the Fire Nation, an inkblot near someone's signature on an important document showed that the person was entering into the contract unwillingly or in bad faith. "A bad omen indeed."
Finishing off his latest glass of wine, Zuko wished to any spirit who might listen that he could return to the beginning of this whole sordid story. Maybe it would end differently?