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

------------------------------------------

Sans understood it now, why his father had been so damn anxious, when it came to his own wedding day, with that flame monster - Grillby. How mind blowing it was to think, that it was now his and Frisk's wedding day that was actually here, and it was even more insane that the ceremony had already started. Speaking of Grillby and Gaster, it had been decided that they would actually walk with Sans - especially because it was what his father had wanted. 

"I'm so proud of you, Sans..." Gaster decided to quietly say to him, before the skeleton would have to separate from him and the flame monster, to step up to the front of the room. Gaster grinned widely. "Your mother would be proud of you too." He mentioned, while Grillby nodded in agreement, knowing he agreed with his husband. 

"She would be." Grillby spoke up. "As am I. Both as your best friend, and..." He wanted to say that in a way, he was kind of like Sans' second dad at this point, though he felt too sheepish to say it. Sans chuckled. 

"i know... thanks. both of ya..." And to mom. If she was watching, he could almost imagine her smiling down at him now-- if there was an afterlife. But even his cynical mindset couldn't upset his mood today, as both the skeleton and the man made of flame went to sit down, while he went to stand in his rightful spot, nervously wringing his hands. 

Entering after him, it was Papyrus, holding both Leo and Luna's hands, as they were all dressed up in their cute little outfits. Sans stared back towards his brother, who appeared to also share the same proud glint within his eyes. Papyrus was glad for his older brother, that he had managed to find himself someone who made him genuinely happy. Someone who made live even more worth living. Someone who he had decided he would actually want to spend the rest of his life with. Sans deserved to be happy, that's how Papyrus, (and everyone else) felt. 

After that, it was Andrew walking in with Blair, (as a groomsman and a bridesmaid). To tell the complete and honest truth, Andrew had still been a little caught off guard that Sans had even offered that he could be a groomsman, even if in recent times they had been off to much better terms. They hadn't been the greatest of friends, and hadn't always agreed on things. Sans had asked him more for Frisk than himself, but still hadn't minded asking him. The hate he had once had for Andrew had long faded. He couldn't care about that anymore. 

Then of course, it was Chara and Asriel. Chara had been a bridesmaid at Gaster and Grillby's wedding, alongside Frisk. Of course, when she was asked by Frisk to be a part of this one, it wasn't like she could refuse. Frisk was like her sister after all, always had been since they started spending time together. So, she was more than honored to be able to attend her wedding like this.

Okay... Okay, this was actually it. This was actually freaking it! Seeing everyone stand from their seats, with the music in the background, Sans could feel the nervousness grow inside of him. It wasn't difficult to spot Kai in the audience, who looked on the verge of tears already, and the bride hadn't even walked in yet. Speaking of which, Sans' pupils fell towards the door, as he spotted three figures ready to walk down the aisle. It was the two goats, dressed up smartly for the wedding, with Frisk standing in the middle of them, with the dress she picked, her hair neatly done up in a bun, with her veil hanging over the back of her hair. She was also holding a bouquet of yellow flowers, as they were her favourite.

Woah... How was it even after all these years, this woman could still surprise him with the amount of beauty she could show off to the world? No matter what, she would somehow still catch him off guard, wowing him with how... Glamorous she looked. How elegant, how sweet... Once again, his mind went back to a thought he had thought a long time ago about her, before they were even going out... It was something that stuck with him for a very long time.

She looked like a princess.

Each goat parent kissed Frisk on either cheek, before removing their arms that were once linked within hers, so she could join Sans up at the front. She quietly giggled, seeing the way he was staring at her. His expression clearly showed how amazed he was, how he was speechless at her outstanding amount of beauty. That shock, slowly transitioned into pure joy, as he actually had to blink back a couple of his own tears. He hoped to be able to hold himself together for the rest of this thing, before bawling like a baby. 

The skeleton could hardly wait for the point that he would be able to say his vowels. He had actually wanted to try and put a lot of thought into what he could possibly say to her, and it had definitely been hard. Because he knew that if he had to put the amount of love he had for her in words, he would just be rambling on and on for eternity. He wondered what Frisk wanted to say to him, as it was coming to the point they were able to proceed to their vowels, and Frisk turned towards Chara and Blair to be handed her paper that had her vowels written upon them, while she gave them the bouquet to hold in return.

"Sans..." Frisk decided to start, as she held the piece of paper within her two shaky hands, as she could feel the adrenaline coursing through her right now, due to her excitement and happiness. Seeing the skeleton grin at the first thing she said, which was just his name, it was enough to make her giggle a little, before she felt like she could continue speaking. "So, here we are... On our wedding day. Several years later after meeting... In a pretty bizarre way." She laughed a little, while Sans chuckled at the memory. "We hadn't started on the best of terms, especially because I think we were at the worst points of our lives around that time. You were stubborn, and I was... Probably pretty annoying... But I am glad that I stuck around for you. That I knew under that ice-y exterior that you showed off to the world, that there was more to you than what it seemed like. I am glad that I got to know you better, spend time with you, that you eventually let me in, the day I got to start calling you my boyfriend..." She had to pause and wipe her eyes for a second, as she was getting super emotional, thinking over all the memories. 

He gave her an encouraging glance, as even he blinked back a couple tears of his own. "I've got to experience so much with you that I had thought I had forever lost, alongside my parents. You gave me so much, and I just... I'm glad I met you. I hope we get to stay together forever, that we grow old together. Because you are my soulmate. I love you, Sans." She finished, wiping her eyes a little more as it had felt impossible not to start crying through her words. Good thing she had decided that she would have waterproof eye make-up on, because otherwise she would have been such a mess right about now. 

Being handed his own piece of paper with his own vowels on it, by his awesome brother, Sans knew he didn't want to waste any more time to just say what he wanted to say. "i'd say, life has been a bit of a rollercoaster... with many down moments. that was before I met you, and I have to say, meetin' ya, was probably the best surprise that's happened to me in a long time." He said, glancing up from the paper every now and then to allow his pupils to meet with her two beautiful eyes. His own cheekbones lightly blushed, seeing how much she clearly loved him in return, just from those two eyes of hers. 

"to answer what ya said earlier, yeah, at first, ya were annoyin'." He lightly teased, deciding to go off script for a minute, just to joke with her, which earned a small giggle out of her. "but i'm glad ya were... that ya never gave up on me, even when i gave ya such a difficult time. lookin' back on those moments, sometimes i feel like i don't deserve such a wonderful person. that ya are too good for me. but then i also know i would never want to be with anyone else. ya gave me a reason to keep tryin' again, to not give up on life, to know that there was hope... ya made me realise what it was like to trust again - that it wasn't such a big scary monster... ya allowed me the chance to have a family, which is also somethin' i thought i'd never have." Sans grinned. "i thought gettin' married would be one of the hardest things i'd ever have to do... that it would be too scary... but as i stand here with ya, i know that it's what i want for us. even with some of the difficult moments we've had to climb over, i know i want to stay with ya forever too, frisk. i mean that, with all my soul. forever and always." He finished, letting out a huge sigh, as it had taken everything in him not to sob like a baby, with how happy he was right now. Frisk wasn't much better off, as she was still crying a few happy tears herself here and there.

"I now pronounce you husband... And wife. You may kiss the bride."

Hearing those words, Sans edged closer to Frisk, going to carefully put his arms around her as he closed his eyes and softly kissed her, while she gladly proceeded to do the same, feeling her heart utterly swoon for this skeleton all over again, like when she had fallen in love with him for the first time. They could hear the applause from the others, but for a second, as they broke away from the kiss and beamed at each other with tearful eyes, it was like they were in their own little world right now, saying back and forth in whispers about how much they adored each other. How happy they were, that they'd get to spend the rest of their future together. 

As husband and wife.