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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Not only had it ended up being a busy day for Frisk, but it just seemed like it was one of those busy days for everyone. Seeing as Sans also was at work, Papyrus and Mettaton had volunteered to babysit for the two of them, looking after the two little toddlers who currently wouldn't be at kindergarten for the day, seeing as it was a Sunday. The pair never minded looking after the two though, (especially Mettaton when it meant he got to spend a couple of hours in the same house as Papyrus... Of course). But it was also just nice looking after the two little ones who were surprisingly well behaved for their age. 

"Uncwle Paprus!" Luna blurted to her uncle who was currently sitting on the couch with Mettaton. When Luna spoke, it was always the sweetest thing, because she wasn't always able to pronounce words properly, and it sounded so darn cute in her angelic voice. "I drew you a picwure." Saying that, she held up a piece of paper to him which had a doodle of Papyrus, but it wasn't just Papyrus by himself. Luna had decided to draw Papyrus with Mettaton and they were holding hands. "It's of you and Metton." She beamed proudly, feeling like she had made the greatest art piece ever. 

"Awh, that's adorable, darling!" Mettaton complimented the small girl who had her brown hair tied into pigtails. Luna's sockets looked like they were practically sparkling with joy at the robot's kind remark, and at the extremely happy look on her uncle's face as he lightly took the drawing from her to allow his eyes to look over it. 

"I LOVE IT! YOU CAPTURED OUR LIKENESS VERY WELL!" Papyrus said, making Luna feel even more good about herself. She looked up to Papyrus a lot, because he was a really great uncle, so when she got kind comments from him, it made her really happy. And she really liked Mettaton too, so him saying nice stuff always made her feel all warm and fuzzy too. "WHAT ARE YOU DRAWING OVER THERE, LEO?" Papyrus asked from the couch, as Leo was still busy leaning on a small table in the middle of the living room to draw. 

"I'm not saying yet. You'll see it when it's finished." Leo pouted, using one of his arms to cover the drawing up so that no one could peak a look at it before it was done. Papyrus laughed a little at Leo's comment, but was happy to let him finish his drawing before he would see it. Just as this was happening, the sound of the door unlocking could be heard by the two adults. It was pretty evident someone had entered the house when they heard the door close, and none other than Sans came into the living room.

"Daddy!" Luna called happily, practically skipping up to him. "Up!" Luna said, reaching out to her dad, and it was always obvious what she wanted when she said that. She always liked to be picked up whenever Sans came home. Though, he didn't always do it because he was sometimes tired. But when he saw that happy look upon her face, sometimes it was difficult to decline. So with a tired smile, Sans picked Luna up into his arms and she let out a giggle, glad that he didn't say no. "I missed you!" 

"sorry, i missed ya too. it was a long day at work today." He said, then going to put her back down on the ground. "is frisk not back yet?" He questioned, only for Mettaton and Papyrus to shake their heads. He sighed, feeling a tad disappointed he was back before her, only because it was nice to see her face when he got home from an exhausting day. Though, it still felt good to see his two children who had been kept safe in the presence of Mettaton and Papyrus. It took him a really long time to get used to the idea of them being babysat, even if he did trust his brother and Mettaton. He was super overprotective of them since the day they were born, and he didn't think that was something that was ever going to change. 

"Mommy is still at work. But look! I made a picwure!" Luna pointed towards the picture she had handed to the other two, and Sans went to peer at it, and just like Papyrus and Mettaton, he smiled widely at it, finding it really sweet. 

"it's cute... i still wonder why ya two haven't made it official." Sans smirked towards his brother and the robot. After all, it had been, what? At least a couple of years now that the two had been flirting back and forth now and again, even Sans noticed it. Yet for some reason, the two of them still weren't a couple, and it was as if they were just taking their sweet time. However, he regretted saying such a thing, because the response he ended up getting back just made him exhale, as it had been a thing he wouldn't stop getting asked since Luna and Leo were born. 

"How come you haven't asked Frisk to marry you yet?" Mettaton asked in return.

"What does that mean?" Leo asked from where he was doodling. One of the adults seemed ready to answer him, but Luna, being the bright and bubbly six year old that she was, was already a step ahead of everyone else and decided to answer his question for him. In fact, just the idea of answering his question seemed to excite her. 

"It means that mommy would get to wear a vewy pretty dress, and she would look like a pwincess!" Luna clapped excitedly, already rushing back to her blank pieces of paper on the floor to start drawing Frisk in a really beautiful wedding dress. Sans weakly smiled at her enthusiasm, but then he looked back at Papyrus and Mettaton who looked like they were expecting an answer. Instead, he headed towards the kitchen, but that just caused the robot to follow him.

"Do you not like the idea of marriage or something?" Mettaton pondered, causing Sans to grumble and ask him not to follow him around. He was tired, and he just wasn't in the mood to be pestered about such a thing about why he hadn't got down on one knee already. Half the time he felt like it was a bigger deal to everyone else around them than it was to him. "You love Frisk, don't you? You've been together for six, well, almost seven years. You've had kids together... I just would have thought you would have done it by now." 

"i just don't think it's that big of a deal." Sans admitted, seeing Mettaton give him a look that really questioned what he was saying, which caused the skeleton to grumble in frustration. "don't give me that look. it's not that i love frisk any less just cause i'm not obsessed with marriage. but things are fine how they are. marriage sometimes just complicates things." Sans went on, and when Mettaton asked how, he continued. "well... people get divorced. it's more so that i love her so much that i don't wanna cause things to change. because then if they change..." He trailed off and just shrugged, not really even that sure of what he was going on about. He just liked things the way they were and he was worried that if things suddenly became different, it might fuck everything up. 

"...Have you even thought to ask how Frisk feels about marriage? Because I have." Mettaton said, seeing the skeleton look at him with a reaction of shock. "She tells me that she probably wouldn't mind getting married one day, but she doesn't mind if it doesn't happen. Only because I think she doesn't want to rush you into anything... Darling, just because you guys would be married, wouldn't mean that things would suddenly become weird. Frisk adores you, clearly. But part of me feels that she would be disappointed if you never asked her. She just doesn't want to be the one to say it." The robot said. Because ever since he asked Frisk what her thoughts were and she said she didn't mind, he just remembered the slight hint of sadness in what she said, and it was clear that getting married to Sans was very likely something she actually wanted. He suggested that she should ask him instead, but again, she didn't want to frighten him. 

"All I'm saying is, perhaps reconsider that thought." Was the last thing the robot commented, before he went to leave the kitchen and leave Sans alone to his thoughts. Marriage, huh? He still wasn't totally sure. True, when his mom had been alive, she had been married to his father, and those two were still as close as anything. He hardly remembered them bickering, and when they did, their arguments were always solved with sweet apologies, and they were always back to being a happy couple again. He didn't remember them arguing on a regular, they had been very close, but when that did happen, it wasn't like it was the end of the world.

But then you looked at couples like... Asgore and Toriel for example. The fact that they were even a married couple still surprised Sans sometimes. Lately, it did feel like Asgore was trying more and more to be there for his wife and his family, but sometimes Sans just imagined that any day now those two would be on the brink of a divorce. He had heard that at one point those two had been practically inseparable, but they were so much different now. What if that became him and Frisk? He didn't want that. 

"I'm home!" Frisk's voice called, after stepping through the door, which caused him to break away from his thoughts. He smiled a little at the fact that she was now home, and he could excitedly hear Luna once more squealing that her mom was home. Did Frisk really want him to ask her to marry him that much, or was Mettaton just trying his best to convince him to think the opposite to his normal thoughts. He shook his head and sighed, not wanting to overthink it. 

And that's when he went to leave the kitchen to greet his loving girlfriend.