Sans is a MONSTER and not a HUMAN, so he doesn't follow HUMAN biology like a HUMAN skeleton. Same with Gaster and Papyrus.   

I suck at writing weddings xD

ANYWAY

ENJOY

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"i never imagined ya for the nervous type, dad." Sans couldn't help but jokingly state, as he was leaning against the doorframe to Gaster's dressing room, while seeing his father anxiously staring back at himself in a mirror, wearing a nice, clean suit. The day for Gaster and Grillby's wedding had sped by extremely quickly, so much that Sans and Frisk hadn't been a couple for at least a couple of weeks. Sans had been trying his best to see the kids at every opportunity, he even gladly looked after them when Frisk had her time taken up by work. However, the atmosphere between him and his ex was still extremely uncomfortable. Sans didn't really know how to gain the nerve to just say: okay, whatever. To just accept that she was the boss of the Mafia, and that he would put up with it. 

"I look okay, right?" Gaster wondered, turning towards his son, just as Papyrus was showing up, standing behind Sans. Papyrus' face lit up, as he went ahead to exclaim that their father looked extremely smart. "You really think so?" Normally, Gaster didn't really care so much about his dress sense, especially because he practically wore the same outfit whenever he was at work. He dressed in black and white from head to toe constantly, so why did he not feel used to standing here in a black suit? Maybe it was just the nerves of what day it was. 

"papyrus is right. grillby will be amazed." Sans grinned. "what are ya doin' here anyway, bro?" Sans questioned, glancing towards Papyrus. 

"WELL... GRILLBY IS THE ONE WALKING DOWN THE AISLE RIGHT? SO I WAS COMING BY TO CHECK YOU WERE READY, SO YOU COULD STAND, WAITING FOR HIM." Papyrus explained, while Gaster nodded, having his back turned to the two, but he could clearly see his two sons standing in the doorway when he stared at the mirror. "SO WHENEVER YOU'RE READY..." Papyrus continued, not wanting to pressure Gaster too much. He was probably the most nervous the brothers had ever seen him, but that was probably no surprise. Sighing, Gaster fought to regain his composure, and then turned back to the two boys. 

"It's okay, I'm ready." Gaster nodded, earning smiles from the two brothers, who were extremely happy for their dad. Weddings were always supposed to be the happiest day of a couple's life, so it was nice to see their father so happy. Because they were having the wedding in a really nice hotel, all they had to do was walk down a couple of hallways to get to the large room where the ceremony would begin. Rather than walking down the aisle during the wedding, Gaster had decided that he would already be stood, waiting. Especially because he didn't have any parents to walk with him. Sans and Papyrus also decided to stand to the side, at the front, staying near their father, while people began to gradually file in. 

"This is pretty exciting." Blair murmured to Andrew, as they went to take their seats within the group of people that were just watching the ceremony. "I've never been to a wedding before." Blair admitted quietly, as it was time for the two people who had volunteered to be bridesmaids to walk in. The two who had actually decided to be bridesmaids when accepted by Gaster and Grillby was Chara and Frisk. At first, Chara didn't really know if she had liked the idea, because she wasn't a huge wedding person. But then of course, Frisk had to play the Asriel card, mentioning how if he saw her walking down the middle of an aisle in a pretty dress, there was the possibility that it wouldn't only be Gaster and Grillby's wedding they'd be attending.

Chara had found that comment so stupid... But was still somehow convinced. Just... The thought of perhaps catching Asriel's attention, as he was sitting and watching the wedding happen, it was something that meant a lot to her. Because yeah, she still cared a lot about that stupid goat. She was just too nervous to say it. 

Anyway, with small smiles of their own, both Frisk and Chara began to make their way into the room, down the aisle so they could also stand near the front. Sans would be utterly lying, if he had said Frisk hadn't caught his eye, in that stunning blood red dress of hers, and with her hair neatly done up in a bun. He was still crazily in love with her, so how could her appearance not have such an effect over him, as he honestly felt his soul swoon for her. For a second, the human couldn't help but notice the skeleton staring, as if he was trying to catch flies in his mouth, (seeing as his mouth was hung slightly agape), and she could only flash him a awkward smile, before ending up in her spot at the front. 

Damn... Sans knew that he would be back to being crazily jealous, if she ended up with someone else. Even when he knew that he was finding it difficult to decide to put their relationship back together, seeing her with someone else for a second time, would just be soul wrenching. She was such an elegant woman, she practically glowed. Dammit... Why did his emotions have to be so conflicted with one another? He wanted to be with her, but then he didn't think that was a good idea. Curse him and his overthinking mind.

Lastly, but not least, everyone decided to stand from their seats, as the gentle music began to play, signalling that Grillby would soon be walking in. Standing with his hand linked onto his wrist in front of him, Gaster stared towards the doorway expectantly, only to softly smile, and feel like he was falling in love all over again, as he saw Grillby step into the room, wearing a clean, black suit of his own. He looked so extremely handsome, and Gaster hoped Grillby knew that's exactly what he thought, just by the look in his eyes alone.

"Mr Grillby looks really nice." Luna whispered to her brother Leo, who nodded in agreement, as they were sitting with Toriel and Asgore. 

"You look amazing..." Gaster decided to whisper, once Grillby had joined him at the front of the room, chuckling a little as he lightly blushed a shade of blue at his cheeks, due to Gaster's kind compliment. The two honestly were just so busy staring at each other with so much love, that they almost missed when the person who was leading the ceremony, had said they were allowed to say their vows. Grillby decided he would go first, as he thanked Chara for handing him the piece of paper that had what he wanted to say written down on it. 

"Gaster... The day that I met you, when we were teenagers, I had absolutely no idea that you would end up becoming so important to me. It had been scary at first, feeling like you were going from my best friend, to the person I utterly adored with all my heart. Even if you hadn't seemed to feel the same at the beginning, I had just been happy to be in your presence. I was happy to be there for you. To comfort you, when you needed someone. When you told me you loved me, it was the best feeling in the world. I just know that I want to spend the rest of my life with you... And it..." Grillby had to pause, feeling like he was going to bring himself to tears - happy tears - from saying all of this. It really meant so much to him. "It would mean everything to me, if we get to spend the rest of our lives together." 

When being handed his own vows by Sans, Gaster had to take a second, before he would start speaking. Only because Grillby's words had really touched his soul, and he wanted to be able to make it through it own words without bursting into tears. Smiling widely, Grillby stared tearfully back at Gaster, as he allowed him to say what he wanted to say. "If anyone has ever been there for me throughout almost the entirety of my life, it has been you, Grillby. As teenagers, we struggled through school, and every day life together. As my best friend, you really made me feel better about myself, and what I wanted, when it felt like back then, my parents hadn't been very supportive of me. You had been my escape, and not just in simple situations like that. You were there for me when I lost my first love... Looking back at that, it means so much more to me, that you put your own feelings aside just to comfort me... You never had to do that. Yet you did. Several occasions, you saved my life, especially in moments where I just felt like giving up. You mean the world to me, Grillby. And I love you. I hope we can continue to grow even older together, forever." Gaster tearfully chuckled, earning another tearful laugh out of the flame monster.

After speaking words of love to one another, it was then that they were allowed to give each other their rings. Gaster was the first to take the ring he would give to Grillby, from Papyrus' grasp. Lightly taking Grillby's hand into his own, he gently slipped the wedding ring upon his marriage finger, and it was soon after him, that Grillby would proceed to do the same. "You may kiss the groom." With that being said, the pair wrapped their arms around each other in affection, and gave each other a loving kiss. Everyone watching began to clap, with some people even starting to cry here and there. 

It had been such a wonderful ceremony.