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

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

The after party... It seemed like to Sans, whenever he was at a party that involved dancing, or socialising with others, he always had extremely bad luck. It was already an hour or so after the main ceremony had happened, and they had moved to another part of the hotel, where there was a dance floor, and a DJ, and tables were laid out across the room, to allow people to sit and eat. Currently, Sans was sitting all alone, as everyone else who could have possibly occupied his table were up dancing on the dance floor. Chara was dancing with Asriel, seeming to have a blast, Blair was dancing with Andrew, Toriel was dancing with Asgore, Frisk was spending time dancing with Leo and Luna, causing them to be giggly messes all the while, Papyrus was dancing with Mettaton... And on and on...

Such a thing truthfully gave him deja vu back to several years ago. No, it wasn't like Sans was seeing Frisk dance with another guy, while he was forced to stand at a distance... But it still drove him insane, knowing how much he just wanted to hold her, and be close to her, yet his mind wanted to be so fucking annoying. He couldn't decide what he wanted. He really, really couldn't. With an exhale of breath, Sans decided he might go outside for a smoke, feeling like if he did that, he might be able to relax a little more at this party. It did mean he would have to go to the steps, that were at the front of the hotel to even have a smoke, seeing as this hotel did not allow smoking inside, but he didn't care. He just needed a cigarette. 

From where she was dancing, Frisk couldn't help but notice Sans seeming to head towards the door, as he was headed somewhere. Frowning, Frisk thought for a moment, before she knelt down to Leo and Luna. "Hey you two, I'm going to go get some fresh air for a moment, okay? How about you go join Asgore and Toriel for a bit?" Frisk suggested, while Leo and Luna nodded, going to do just that. Standing upright, Frisk was already on her way to follow after the skeleton, until she heard Gaster calling after her as she was close to the door, stopping her in her tracks. She turned to face him. "Yes, Gaster?" 

"Sorry, I do not mean to stop you when you're in a rush. I noticed you seemed to be following after Sans." Gaster smiled, seeing Frisk glance away slightly in embarrassment, realising she had been caught red handed. "You realise that if you're thinking of asking him to get back with you, he won't if you're still the Mafia boss..." Gaster stated, causing her to feel like her heart was sinking, because she knew how true that was. No matter what she could say to him, he would reject her if she was still running the Mafia. "I actually wanted to ask you something. I would offer to take your place, if you want?" 

"W-What?" Frisk looked at Gaster in shock. "N-No, you can't... You have Grillby..." 

"I already talked to him about it." Gaster responded, continuing to smile. "He agreed that I should do it, if I felt like I was capable, and if I wanted to. Truth is, quite a long time ago, I had already said I wanted to be the next Mafia boss. But so much happened since then, that I wasn't so sure. But I'm sure now." Gaster said. "You won't be burdening me, Frisk. Honestly. I promise." The skeleton beamed. 

"I..." Frisk started, not even sure what to say. She didn't know whether or not to accept his offer, to hand the position over to him. Because that's what she was nervous about, throwing it onto someone else who never asked for it. But, Gaster was actually asking her to give the position to him? Would he really want that...? She seemed to think about it, for what felt like an eternity, until suddenly, she stepped forward, and decided to give Gaster a hug. "...Thank you." Frisk said, feeling like she could sob right now. Gaster smiled, happily hugging Frisk in return.

"Any time, Frisk. Now go on, I'm sure there's a grumpy skeleton waiting for you to make his day." Gaster teased, giving Frisk a light push towards the doorway, and with a small laugh, Frisk began to go off into a run, at least to the best of her ability, considering she was wearing heels. She didn't want to waste any time, talking to Sans. She just hoped he would take the time to listen to her. 

Sitting on the steps, out in the night, Sans took a small puff of his cigarette, as his pupils lazily trailed across the dark sky, tracing across the many small dots that glistened, a million miles away. He was still picturing Frisk, in that nice, hot red dress, looking crazily stunning, before letting out a sigh. Honestly, despite the break up being his fault, there was no way to express just how badly he missed Frisk, to being able to say that he loved her, to kiss her... He wanted to do all of that again, but...

"Sans." A female voice spoke behind him, and it was clearly obvious who that voice belonged to. Frisk... He didn't really know what to say, how to answer. Not that she gave him much of a chance to answer, as she was already clip clopping over in her high heels, before deciding to sit down next to him on the step. He decided to edge away a little, not because he hated being near her but... Well, because he was so attracted to her, that just being near her drove him crazy when they weren't a couple. "Hey..." 

"...hey... what are ya doin' out here?" Sans asked, continuing to stare towards the night sky. Frisk sadly smiled at him.

"I noticed you leaving, so I wanted to talk to you..." She murmured, going to glance towards his skeletal hand that was resting on the stone step, next to him. "Sans..." She spoke again, going to put her hand on top of his, but the moment she did, he drew his own hand away. She could have almost sworn a guilty look spread across his face, as he didn't want to look like a prick, but... "...You know how much I love you, don't you?"

"this conversation again...?"

"But you do, don't you?" Frisk repeated, waiting for the skeleton's answer. Sans hesitated, taking another puff of his cigarette, until he quietly responded with a almost silent: yeah... She beamed. "I've never stopped loving you, since we broke up... I've so badly been wanting to talk to you, but it's never felt like the right time, or place..." Frisk sighed, going to stare forward, as she wrapped her arms around herself, both because it was cold out here, and because she was nervous. It almost felt like the first time they had confessed to each other all over again. "You were angry with me, because too much of my time would get taken up by the Mafia... Because I failed to keep my promise, when I said I would quit the Mafia..."

"i still am mad." Sans added. 

"I don't blame you... But, Sans... I just spoke to Gaster, literally just a second ago and... I'm not the Mafia boss anymore." She beamed, instantly seeing Sans glance towards her out of the corner of her eye, when she said that. Knowing he was staring at her with eyes wide with shock, she decided to return the stare. 

"Him and Grillby had agreed that he should take it... So..." Frisk shrugged. "You mean the world to me, Sans... Every day, going without you, sleeping in our bed without you... It was painful. It's hard, trying to be happy, when I know I'm not. Without you, I don't think I could be happy ever again." She confessed, slowly rubbing her hands together, both to keep them warm and due to her anxiousness. 

"I'm sorry that I let you down, again and again... I'm sorry that I haven't always been the best mom, or the best partner, but I hope you can believe me when I say that I'll do better. I will quit the Mafia for real this time, and if you want, you can always go back to your job, to the Mafia. I'll see if I can work things out with Kai, to see if I can work in the cafe again." She softly smiled, before taking the ring off of her finger, that was supposed to belong to Sans, and she held it out to him. 

"Please, Sans... I'm begging for you to forgive me. Because I just feel like life would never be the same without you. I love you, and I want to remain yours forever. I want to become your wife, to continue looking after our kids together. I promise, you and the kids are the most important thing in the world to me..." Frisk whispered, blinking back her tears. "So..." She trailed off, as her pupils slowly went to the ground that was in view between her and Sans. For a couple of seconds, Sans was completely silent. He didn't really move, or do anything. At least, until he appeared to throw away his cigarette. Then, using one of his hands, he cupped the hand that was holding the ring within his own, until he leaned towards her, giving her a quick peck on the mouth.

After that first peck, the skeleton backed away, only a little, but it was at that point that their eyes were locked, and everything seemed like it wanted to go into slow motion. Sans moved back in, pressing his mouth back to Frisk's lips in a more heated, affectionate kiss, as he wrapped his arms right around her waist. Fluttering her eyes closed, Frisk went to wrap her arms around the back of Sans' spine, wanting to be as close to him as she could possibly be, after all the time they had spent apart. 

"i love ya... god, i love ya." Sans spoke softly, when he had a second to pull away to catch his breath, only to kiss Frisk once again. There was no simple way to describe just how badly he had missed her, how badly he had wanted to be near her, to tell her he adored her, to be able to lovingly caress her sides with his hands while he kissed her. For awhile, the two just continued to keep telling each other back and forth, how much they cared about one another, as they kept kissing. At least, until they were interrupted by someone clearing their throat.

"I hope I'm not interrupting too much..." Chara stated, having her hands on her hips as she stared at the two. "I didn't know that you two were bold enough to try making babies outside." The girl teased with a smirk, instantly seeing the two blush madly from embarrassment, while they tried to stutter and explain their way out of it. Chara chuckled. "I'm just joking... I'm happy you guys seem like you have made up! Gaster just told me to fetch you two, seeing as it's almost time for Gaster and Grillby's dance... But... If you want to continue with whatever you were doing..." She trailed off, leaving the rest of what she was going to say up to the imagination, before she left to go back inside.

Embarrassed, Sans went to stand up, before offering his hand out to Frisk to also help her up. Letting out a small laugh, Frisk happily accepted his hand, as she pulled herself up. "...Here." Frisk was still holding onto the red ring, offering it to him. With a tiny grin, the skeleton held out his hand to her, and gladly allowed her to slip the ring back onto his finger, before he hugged her tight. "I love you..."

"i love ya too... i'm sorry... i'm so sorry..." Sans apologised, hugging her tightly as if he was afraid to let go of her, like she would just disappear if he did that. "i'll never let ya go again." He promised, while she promised the same in return. The last couple of weeks where they had been apart, had been complete and utter torture. He was just glad it hadn't lasted any longer, like a couple of months, or even years. He wasn't sure he could have handled that. Because Frisk was a part of him. She was his future wife. 

He had already known a long time ago, that he wanted to be with her forever.