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

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

What was sweet, was how the moment Luna had seen her little brother getting upset, she came on to usher him away from the staring eyes of parents, and Sans had gotten up from where he was sitting, knowing that he wanted to take them home right away. There was no way in hell Sans was forcing Leo to go on with the play if he didn't want to, even if he was one of the main parts. Thankfully, the teacher of the class seemed to understand, and gladly allowed him to take his children home; there was no way Leo would have been able to perform, being in pieces.

When he had taken Leo and Luna home, almost the minute they had gotten through the door, (as he had been carrying Leo the entire time), he went to place the child down on the couch, before grabbing a blanket to wrap right around him, and then he sat down next to him, pulling his son right into his arms to hold him close. Feeling sorry for her little brother who was still sobbing, Luna climbed up onto the couch to join in on the hug, so Sans happily held her close too. 

"W-Why didn't mommy show up...?" Leo quietly asked through his tears, which made Sans frown. "She said she was excited..." Saying that, Leo began to cry harder, and Sans could only gingerly squeeze him in his arms, as if to try and relax him a little. Sans knew Frisk likely didn't mean to not show up; she had been seriously excited to be there; that didn't mean he was any less angry. 

"she didn't mean it, leo... mommy loves ya, she really does. she probably just... got held up." Other than that, Sans really didn't know what else to tell him. Despite his rage, he didn't want to let his children think that Frisk was a bad mother. Because... She wasn't. She was just... He didn't know. He really didn't know. He just didn't want his kids to be upset, he wanted them to be happy, and they weren't going to be happy getting angry with Frisk like he was. Even if he was pissed at her, he didn't want their children to hate her. 

"Yeah! Mommy didn't mean it." Luna put in, as she wasn't really angry at their mom. She knew she probably had her reasons for not having shown up. But despite their words to try and cheer the youngest skeleton up, he was very clearly still upset. He had hated wanting to do this for a very good reason, he should have just stuck by his word when he decided he didn't want to be a part of it; then he wouldn't have ended up being humiliated in front of his whole class and all their parents because he began to cry. As if people didn't have enough reasons to not be friends with him, now his friend Kyle was probably going to not want to be friends with him either, because he was just a crybaby. 

Hearing a knock at the door, Sans went to get up, telling Luna to stay with Leo and to make sure she tried to cheer him up. Of course, she happily accepted, while Sans went to get the door, only to narrow his sockets in anger when he saw just who it was. Frisk. "how nice of ya to show up." Sans growled, as the last two words he said had a slight bite to them, as he clearly was furious. "kitchen. now." Sans demanded, before she even got a second to explain herself. Stepping aside, he allowed Frisk in, to direct her towards the kitchen, so they could have their discussion out of the way of their children, even though it was inevitable they were probably going to hear some of the conversation anyway. He would rather not argue with Frisk in front of them.

"alright, i want a good damn fuckin' reason why ya weren't there today." Sans told her, crossing his arms as he glared towards her. Even with that guilty look on her face, he didn't falter, as he wasn't going to let her off so easy, not like the bunch of times he had already done that. "i've given ya multiple fuckin' chances. i really didn't think ya could fuck up, again." Sans ranted, knowing that if he was a human, by now he would have a vein in his head that would be about to pop. 

"Sans, I'm sorry--" 

"i don't think it's me ya should be apologisin' to." Sans interrupted her, as the pupils disappeared from his sockets. He may have been angry, and he may have been hurt, but he didn't feel like he was the main priority right now. Not when he saw that look on Leo's face. "the last thing i ever wanted to see, was seein' leo hurt, because his mom, after makin' him feel excited to see her there, didn't show up." He went on. "he broke down cryin', frisk. so tell me, the excuse better be fuckin' good." 

Frisk stared back at the flaming skeleton, who really looked ready to snap. She thought of what to say, until she sighed, and glanced away with sorrow. "I don't have a good excuse... Not one that you would want to hear, anyway." Frisk sadly spoke. "I'm sorry, okay? I'm sorry to Leo, and I'm sorry to you. Because I know you're hurt, and I don't want to be the person hurting you." She really didn't like seeing that look upon Sans' face. It was a face she hadn't seen in quite a long time, one that he might have used more during the time before they were even together. She just knew he was mad when he was looking at her like that, with the pupils completely gone from his sockets.

"well, here's the question. are ya gonna quit, or not?" Sans demanded. "because if ya even have to think about it, then..." He didn't want to say it. He really, really didn't. Had he not trailed off, then the end of that sentence would likely have been something like: then I'll walk. Because here was the thing, he didn't want to break up with Frisk, he didn't want things to end between them, and he wanted to believe that things would be okay, that they could work this out. If this was going to keep happening over and over, where she couldn't choose between her family and her job, then... Maybe it just wouldn't work out. 

"Sans, please don't make me choose." 

"so ya do have to think about it?!" Sans snapped. 

"You would say that in my position you would gladly drop a bunch of people, allowing them to just lose their jobs?" 

"yes!" Sans shouted at her, not even hesitating to answer her question. Due to the fact he had raised his voice quite a bit, she was a tad startled by his yelling, but he didn't care. Instead, he continued with what he was saying, because if he had to drill it into Frisk's head, then he would. "because to me, my family is more fuckin' important! ya would give me shit if i missed somethin' that was a big deal to our children, so do ya know what i did? i did the right thing. i shaped up, listened to ya, tried even harder to be a better dad. i didn't make the same fuckin' mistakes over and fuckin' over, like a repeatin' fuckin' record!" Sans ranted on, trying very hard to blink back the tears that were forming in the corner of his sockets.

"to me, losin' ya was more important than any damn job. i could give less of a shit about work, if it meant i could stay with ya and the kids forever." Sans told her honestly, while she remained silent, not knowing how to reply while he was going on his furious tirade. Instead, she thought it was better to not say a word, allowing him to get out what he wanted to say. A tiny part of her wanted to argue her position, but she wasn't sure it would make it any better. "but i can clearly see ya don't seem to think the same, if ya even have to hesitate about what ya would rather choose!" 

"Sans--" 

"just tell me right now. are ya gonna quit, or not?" Sans pressed, still staring at her with those two empty sockets of his. When she didn't seem to answer, and she glanced away, he could only exhale, turning his back on her for a second. It was then that he couldn't hold his own tears back, and he started to cry. At this point, Frisk wasn't sure whether to step towards him to calm him down, or to stay rooted where she was. With Sans being angry at her, she felt like trying to calm him might only make it worse. He slowly shook his head, his back still turned to her. "i can't believe this..." 

He thought that Frisk of all people would understand his upset, that she would have just listened to him when he told her to stop with all of this madness. But she wasn't listening, was she? Even now, she still wasn't going to quit. How unbelievable. "fine..." Sans sniffled, going to wipe away his tears, before facing back towards Frisk, who looked at him with a mix of confusion and sadness. He still had that angry look on his face, but his expression also held heartbreak. "if ya can't decide what ya would rather focus on, then let me help ya." Sans snarled. 

Watching him in bewilderment, Frisk saw Sans walk right up to her, bringing them face to face. She didn't know if he was expecting her to say something, or do something, or what. Instead, she let out a small startled squeak, as Sans seemed to grab one of her arms and he harshly pulled it towards him. It didn't hurt or anything, it had just been a surprise for him to do that. Frisk continued watching him, as he turned her palm around so it was facing up towards the ceiling and to her surprise, he dropped what appeared to be a ring with a red gem on it in her open palm. Wait, when had he...?

Her eyes went from the ring in her palm to his eyes in disbelief. The pupils had returned to his sockets, and he had a look on his face that looked as if he was trying to resist breaking down crying again. But he had to remain strong. "S-Sans?" 

The skeleton didn't really know what to say to her, with that look in her eyes, as it was just clicking with her what he was saying. He had taken off his engagement ring, and he had given it back to her, because he just knew that if they were  going to keep arguing back and forth like this, if he was going to have to keep pretending he was okay when he wasn't, then maybe it just wasn't going to work. It was then that Frisk started to cry, which set him off too. He didn't want to fucking do this. But if she couldn't make the decision to pick between one or the other, then... 

"i'm gonna stay with dad for awhile. ya can stay here with the kids. make it up to them. and another thing - i quit." He told her, before turning around to go for the front door. But before he had hardly got the chance to reach it, the kids had gotten up to speak to him.

"I want to go with daddy!" Leo said, which made Sans turn around to look at the small child. He shook his head. 

"leo. ya should stay here with mommy." 

"No!" Leo yelled, not caring that Frisk was standing in the doorway of the kitchen, watching the discussion. The 6 year old went to cling to his leg, yelling about how he wanted to go with his dad, while Luna on the other hand looked conflicted. She wanted to go with Leo and her dad, but then she wanted to stay with Frisk. Seeing that look on Luna's face really broke Sans up inside. He sighed.

"luna, if ya wanna stay here, it's okay... there's no reason to feel bad, alright?" Sans told her softly, while she slowly and sadly nodded. That being said, he reached towards her to hug her, and kiss her on top of her head. "it'll be okay." He told her, giving her a weak smile, before he stood upright and looked towards Frisk, who still hadn't spoken a word since he dropped the ring in her hand. He didn't really know what to say to her. 

Because Sans literally couldn't pry the other 6 year old from his leg, with Frisk's permission, Leo ended up going with the skeleton, knowing he would rather stay with his dad than his mom right now. Not that Sans really approved, as he thought they should both stay with Frisk. But Sans couldn't get the child to agree to do that. That being said, Sans ended up driving him and Leo to Gaster's house, in complete silence. He just hoped him and the flame monster didn't mind him and Leo crashing at their place for a little while, until they decided what the hell they were going to do. He didn't even know if this break between him and Frisk would last forever, most of him hoped that it didn't, but if it did... 

It was just their luck that the moment they arrived at his father's house, that it began to piss down with rain. Sans tried his best to shield Leo from the hammering droplets of water, as he knocked upon Gaster's door. They waited for a couple of seconds, until the door was opened, revealing a surprised Grillby. 

"hey... can we come in?"