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

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Why hadn't she just said something? She felt like such an idiot, as back there, she wasn't sure how to tell Sans she was sorry, how to tell him that he really did matter to her. It wasn't that he didn't. What she had wanted to do was that she didn't want to allow people to lose their jobs, and if at some point she could find someone to replace her, then maybe she could do that. But right now, there had been no one else who had wanted to take Asgore's position. She had got so caught up with work that the moment she saw that it was beginning to overlap with Leo and Luna's play, she began to panic. She wanted to quit for Sans, but now that she had made herself the boss for everyone else, she couldn't just do that, could she?

It had been a couple of hours since Sans had left with Leo, and Frisk had tucked Luna into bed. She had to reassure her daughter that everything was going to be okay, but right now she wasn't so sure, as she was sitting at the dining table, with the ring that she had given Sans resting in the palm of her hands, while she softly cried. Did this mean that her and Sans were over then? It obviously meant that the engagement was off, so they must have broken up. There were no words to describe just how torn up she was. All she wanted to do was sob all night, worried that she had lost Sans forever.

At Gaster and Grillby's, it wasn't like Sans was any better off either. The moment they had gotten through the door, Grillby had called to Gaster to fetch Sans and Leo a towel, as they had both been out in the rain. First, Leo was put to bed in a spare room they had in the house, before Sans began to word vomit, explaining everything that had happened to his father, as they sat on the couch, and Sans had the towel wrapped over his shoulders. Even now, the skeleton was still crying, while his father was trying to calm him down, as he was fearful he had completely lost Frisk. He didn't want to be without her. But then he wasn't sure how else to hammer it into her head how badly all this stuff of her getting hurt and her not listening to him was affecting him. 

"Sans... If you two had a calmer discussion with each other, I'm sure you could work it out." Gaster told his son, but telling him that only seemed to make Sans want to break down more. "But if you two really need a break from one another, maybe it's for the best. It doesn't mean it's going to be permanent." After all, it was clear that Sans and Frisk were meant for each other, and likely no one could imagine either of them being with anyone else. Just because they were having a couple of bumps in the road, did not mean that their relationship was suddenly done for. 

"i just... i don't like her bein' in the mafia, dad." Sans murmured. "in fact, i'm not even part of it anymore... i told her i quit." Sans went on, staring down towards the ground with a look of misery plastered across his face. There really wasn't any simple way to describe how he was feeling right now. At the current moment in time, it just felt like there was a million things at once going off inside his skull, and his mind didn't know what to settle on to talk about. "i know it makes me seem like a controllin' asshole, but i just... i don't want her to become me." Sans explained. 

"You're not a bad person. You have been doing much better, since being with Frisk." Gaster said. 

"...and i fucked it all up. heh. that's just typical sans, ain't it?" Sans glumly chuckled; it was obvious his laugh held no humor in it whatsoever. Instead, he sounded extremely depressed. What he had been hoping for, was that the moment he had put his ring back in Frisk's hand, Frisk might speak up and decide that it was better that she wasn't the Mafia boss. But she didn't say a word. Part of it may have been just because he shocked her so much with his action, but it's not like he could just take it back now, could he?

"You're not a 'fuck up', Sans. I understand why you were frustrated." Gaster told him. "When I heard that Frisk was going to be the next boss, I was quite skeptical myself. I really don't think it's for her." He said. "You and Frisk will work it out. But for now, give it time. Not too much, but just enough that the two of you can have a calm, thoughtful discussion about it, okay?" Gaster finished, while Sans quietly nodded. 

"okay..." 

"Get some sleep, alright? You clearly need it." Gaster told him once more, instructing Sans to lie down on the couch, and he went to fetch him a blanket, which he placed atop him. The skeleton honestly looked unsure about trying to go to sleep, especially when Frisk wasn't here. But he had no choice but to sleep alone, after what he had done. He glanced towards his left hand, which normally had the ring upon it, but now that he had given it back, even that couldn't bring him solace. Instead, its lack of presence only made him feel worse, despite him knowing it was his own fault it was gone. 

I'm so sorry, Frisk... That was all Sans could think once Gaster had left the room, and he was left all alone, lying across the couch with his hands held close to his chest. He wanted to talk to her. He wanted to say he was sorry for yelling at her, for getting so fierce with her, but then he didn't have the guts to. All he had the guts for was to lay here and quietly cry, as he was feeling stupidly sorry for himself. Was he in the right? Was he in the wrong? He didn't even know anymore. He just knew he regretted that argument. 

But as much as he wanted to run back to her, part of him questioned if it was even a good idea. Not because he didn't love her anymore, (as he very clearly still did), but just because if they were constantly going to be arguing about whether or not she should be the boss of the Mafia, then he wasn't sure if that was a relationship he wanted to be a part of. He couldn't tell Frisk what to do, so it was completely up to her if she wanted to continue working at the Mafia as their boss. It only meant she would have less time to do anything else, and it would mean that he and her would be less likely to get back together. 

Just thinking that was enough to allow him to cry himself to sleep, as he clung desperately onto the blanket around him, as if it would bring him enough comfort to calm him. But while he and Frisk were apart, it was likely going to be hard for him to feel entirely comfortable about anything. He could try his best to keep on a happy face for his son, who was clearly hurt too, but part of him questioned if he was strong enough to actually be able to do that for Leo. He wanted to continue being a good dad, and he wanted to put his own pain aside for the sake of his son; he hoped he was capable of doing that. 

All he and Frisk ever wanted was the best for their children. They wanted them to lead the best lives they possibly could, better than Frisk had ever had, where she lost her parents, and was forced to live on the streets. Better than Sans had ever had, where he lost his mom and became a paranoid, distrusting mess. Leo and Luna really deserved to be happy, they really, really did. Which was why Sans hoped this split wouldn't last forever. Not even just for the sake of his own feelings, or Frisk's. 

But for their children too.