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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Sans decided to put on a movie for Leo to keep him a little distracted once they had got home, hoping it would be enough to have him not think about how his shortened day at kindergarten went. He also thought he might make some food for the both of them to eat, knowing that he himself right now was feeling somewhat hungry. As he was heading to the kitchen, he couldn't help but smile, seeing two pictures that were pinned up on the fridge. One had been drawn by Luna and it was the picture of Papyrus and Mettaton, and the other was by Leo, and it was a drawing of Sans. But to interrupt his moment of staring at the pictures, he felt his phone buzz in his pocket, and it was a message from Asgore, wondering if he could come into work today. But he simply declined and put his phone on the side, knowing he would rather look after his son today. 

It wasn't like Frisk was a bad mother or anything. In fact, she was a fantastic mother, and it always made him smile the way she was around their two children. He was still so happy to be with her, and he was happy that they were a family together, but sometimes... Sometimes he wished she would just quit. Quit and keep her job at that cafe and live a more normal life. He felt like she deserved that more than this. While thinking this, he heard the sound of the front door, and he heard Frisk's voice say hello to Leo, before she came into the kitchen. 

"welcome home." Sans said with a smile towards her, but then he rose his eyebrows, noticing how a part of one of her arms looked bandaged up. Seeing that, it was no surprise that he was at her side in two seconds. "what happened? are ya okay?" Sans questioned immediately, causing Frisk to giggle a little, as just like always he was already worrying about her from what she considered to be a tiny little thing. It wasn't that big of a deal.

"It's fine. Gaster healed it up the best he could and then bandaged it for safe measure. I got shot in the arm on my mission but... Luckily it wasn't anywhere else." She said, causing Sans to frown. With a sigh, he let go of her arm and went to go back towards one of the kitchen counters. It was pretty easy to notice his sudden change of mood, which now was causing Frisk to furrow her brows a little in confusion towards the skeleton. "What? What is it?" 

"i was just thinkin'... maybe ya should quit. the mafia, i mean." Sans murmured, going to look back at her, seeing more confusion spread across her face. "i don't like ya constantly puttin' yourself in danger." He pointed out. He only wanted the best for her, and like he already established, if she was shot or stabbed or... Something... She was a lot more likely to die from something than he was. Frisk crossed her arms at that, already knowing what she wanted to say. 

"And you think I like you putting yourself in danger?" Frisk demanded. "Why can't you just let me do what I want to do as well? I'm not a kid." Frisk pointed out, causing Sans to growl. "You can't boss me around." 

"it isn't about that!" Sans snapped back, as he rose his voice towards her. He didn't want to give off the vibe that he was trying to control her life; it wasn't like that at all. He knew that he couldn't always tell her what to do, and he couldn't always expect her to listen, but... "i already told ya... i just don't wanna see ya turn into a monster. i've already noticed ya actin' different. and i thought ya didn't like to put work before our kids? ya gave me shit for not bein' around on their first day, and yet ya don't even show up when leo needs ya." Sans complained. Dammit, they shouldn't have been arguing like this. What had gotten into them? They usually never fought like this, ever. Sometimes they had minor disagreements, but... The skeleton exhaled. 

"i just... i'm scared of losin' ya. that's all." Sans finished more quietly, as his pupils trailed glumly towards the ground. He didn't want it to be like another situation where she ended up turning on him, either because of someone else, or because of her own choice. He still adored her with all of his soul, but there was no denying that she wasn't the same person she had been when they were first falling in love. Usually change was a good thing, but he wasn't sure if she was changing for the better.

"...Sans, I love you and our children, more than anything in the world. And I would never even dream for a moment of getting taken away from any of you." She said more softly, staring sadly back at him, even though he wasn't looking at her, and was instead staring down at the floor as if it was the most interesting thing in the world. "I apologise that I couldn't be there today. I never want it to seem like that work is more important to me than you, or Luna, or Leo. Ever." Frisk said, walking closer to him so she was standing right before him. "I love you."

"...i love ya too, frisk. i always will." Sans replied, slowly allowing his pupils to go from the floor to look at her face that held a gentle, soft smile. It was so infectious that he gradually began to smile a tiny bit back in return. "just... let's try not to miss anythin' else that's important to them - either of us." Sans said, seeing Frisk nod in agreement. She couldn't agree more. She didn't want for them to fight and argue with one another because of their jobs, so she would try her best as well to make sure she wouldn't get too taken over by work. 

"I promise. I know it's unfair of me to have a go at you over something, only to not meet the same standard. I'm sorry if I disappointed you today, but it won't happen again." Frisk promised, going to wrap her arms around the skeleton's waist to pull him close and to pull him into a kiss. Sans simply fluttered his eyes closed and put his hands on her waist in return to gently kiss her back. He hoped that she would keep that promise, but he did believe in her, so... He wasn't going to doubt what she said. Frisk wasn't a bad person, she never had been. She might have been a bit different than she had been 6 years ago, but she was still the same lovable girl who he fell pretty hard for. 

"To make it up to you, I'll pick up Luna for today so you can relax." Frisk said, lightly kissing the skeleton once more, this time on the nasal bone. The skeleton let out a small chuckle as he softly blushed, and he was back to being in a better mood now. It was as if they hadn't almost started having a massive argument seconds prior. But the thing was when they argued, it was usually only over something they really cared about, for example: their children. The two of them would never really argue with one other over tiny petty things, like they did when they were first getting to know each other.

"if ya insist." Sans teased back with a grin. "now, as much as i enjoy bein' close to ya like this, i gotta make somethin' to eat." Sans joked. "so ya can go sit down with leo if ya want." The skeleton suggested, knowing she would want at least a little bit of rest after her day at work. So, after earning another quick peck from Frisk, she loosened her arms from around his waist and went to head into the living room to join their son on the couch. 

He was glad that argument didn't get any bigger.