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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"Alright, let us see it!" The skeleton probably shouldn't have been at all shocked that when he and Frisk had returned home, they weren't just greeted by Chara and Asriel like they might have been expecting. While they were gone, either these two had just pranced into their home, or they were invited by the other human and the goat in their absence. It was Mettaton and Papyrus. The only two people who had known that Sans was also going to propose were the two babysitters, so when the robot asked to see the ring, the skeleton assumed he was asking him, so he decided to show them his left hand in which the ring that Frisk had given him rested upon. 

"OH WOWIE!" Papyrus exclaimed, staring at the ring with wonder, but also obvious happiness as he was overjoyed for his older brother, who was slightly finding it difficult not to begin lightly blushing cyan from embarrassment, as for the time being, literally all the attention from the four guests, (Mettaton, Papyrus, Chara and Asriel) was on him. Leo and Luna were currently absent, seeing as it was past their bed time, so they were currently tucked into bed. "I'M SO HAPPY FOR YOU TWO! CONGRATULATIONS!"

"y-yeah, well..." Sans stammered for a moment, before trying to snap out of the fact that he was being gushed over because Frisk proposed to him. "i'm not the only person ya should give attention to, ya know?" Sans pointed out, and when the robot and other skeleton looked at him in confusion, he glanced towards Frisk who widely smiled and also went to outstretch her hand, showing the ring that Sans had given her. Gasps were heard, and all over again the four crowded around Frisk this time, gawking at the ring with the blue gem on it. 

"You proposed too? I didn't think you had it in you." Mettaton teased towards the skeleton, who rolled his eyes at his comment. He was one to talk. Him and Papyrus would always poke at him like he needed to step up and propose, and yet they were constantly flirting back and forth with nothing happening between them. The older skeleton did want to protect his little brother, but he knew that if Papyrus had a thing for the robot, he couldn't really stop him from caring about him, could he? He could only make sure Mettaton treated his brother right, because if that bag of bolts ever did hurt his little brother, he would have to fucking pay for it. 

"says the one." Sans spoke up as he crossed his arms and rose an eyebrow at the robot who seemed to pretend he didn't hear what he said for a second. Papyrus did turn towards him in bewilderment at his comment though. "come on, don't act dumb. i'm all for protectin' my little brother, but if ya two have some kind of a thing for each other, then why aren't ya together? if it's just some kinda fling, then i don't approve." Sans narrowed his eyes at that. After all, he didn't really like the idea of his little brother being a part of a thing where it was kind of like a relationship, but then it wasn't one. Like sex, with no strings attached. 

"Are you really going to put them on the spot like that?" Chara questioned, only to earn a glare from Sans as he pointed out that he would constantly get bugged and put on the spot by people about his relationship. "Okay, that's a fair point." She added, knowing how that was true. "But I've noticed you two flirting around each other, but you've never even made it clear if you're in a relationship." Chara said towards Papyrus and Mettaton who glanced at each other. Papyrus practically looked like a deer caught in headlights, and the robot looked surprisingly calm for all the attention he was getting. 

"Well, first of all, it's not a fling." Mettaton glared towards Sans for a second, at his choice of wording from earlier. Sans simply only gave him the same stare in return, not being at all frightened by the stare the robot gave him. "And second of all... We are already together." He pointed out, seeing a look of shock spread across the older brother's face, as the same happened with everyone else. "We just didn't feel the need to announce it because... Well, why should we have to? We decided to keep it secret since awhile ago, because I didn't want you coming up to me and giving me the talk about how to look after Papyrus. We don't want you to interfere. You may be his older brother, but I already know what I'm doing. I'm not going to hurt him if I love him, am I?" The robot asked rhetorically, while poking Sans in the chest in frustration. The pupils had completely disappeared from Sans' sockets, and Papyrus was afraid his older brother was going to snap. 

"I'M SORRY WE DIDN'T SAY, SANS. WE HAVEN'T EVEN BEEN AN ACTUAL COUPLE FOR THAT LONG." Papyrus admitted, while Frisk went to hold onto one of Sans' hands in case she needed to comfort him or hold him back in case he felt like suddenly pouncing for the robot monster's throat. "BUT WE DO CARE ABOUT EACH OTHER, AND I TRUST HIM. I JUST DIDN'T WANT YOU TO GET ANGRY AT HIM, AND RANT AT HIM TO MAKE SURE HE TOOK CARE OF ME." The younger brother mentioned, going to go and hold onto Mettaton's hand as he said all of this. Sans carefully listened, and over time, (especially when Papyrus when to hold hands with the robot), the skeleton's face began to gradually relax and the pupils reappeared in his sockets. Sans exhaled. 

"...i just want him to look after ya, that's all... it's not that i would have stopped ya from bein' together. all i wanted was for ya to be together if it made ya happy... i just wish ya could have told me." Sans admitted, seeing Papyrus look a little guilty. But then again, like Mettaton said, they didn't really feel the need to announce that they were a thing. Just because he was Papyrus' older brother, it didn't mean he had to be told things all the time, at least not instantly anyway. Sans just shook his head and went to pull his little brother in for a hug. "i'm happy for ya, if it's what ya want." Sans smiled. 

Part of him did feel like he wanted to be super protective of him, seeing as he did miss out on a lot of that when he had tried to avoid him when Papyrus didn't know he was in the Mafia, but he was scared of him finding out. But he didn't want his little brother to be unhappy, and if he would be at his happiest by Mettaton's side then that was perfectly fine. Even if a part of him was nervous about it, that the robot would somehow hurt him or upset him, he had to trust Papyrus' judgement. Papyrus trusted Mettaton enough to love him and be in a relationship with him, so he should do the same. 

"That was eventful." Frisk giggled a little from where she was standing. "I'm really happy for you two though. It is about time you two were a thing." Frisk pointed out towards Mettaton and Papyrus, who once again looked at each other with smiles once Sans had pulled away from the hug, but then Mettaton simply shook his head. As much as he was somewhat enjoying the attention on him and Papyrus, this wasn't really about them.

"It's not about us, darling." Mettaton pointed out, grinning at Sans and Frisk. "It's your night after all. Which was why I actually brought something to celebrate when Chara called us over." The robot went on, motioning for the others to follow him to the kitchen, which they did and the robot had appeared to put several wine glasses out on one of the kitchen counters, which sat beside an unopened wine bottle. Mettaton went to go and pick the bottle up, opening it, only to pour the liquid into the empty wine glasses. "Why don't you each take one."

Doing as he said, everyone in the group grabbed themselves their own glasses; Mettaton was the last to do so. "I thought we could do a toast, as cheesy as that might sound." Chara spoke up, while Asriel nodded in agreement as he stood next to her. Frisk smiled widely at the idea of someone doing a toast, so Chara decided to raise the glass in her hand a little before she started speaking. "To Sans and Frisk, I wish you two all the happiness in the world." She said, before the others joined in on their toast to clank their glasses together. Sans rose his eyebrows a little at her toast, but his mouth could only break into a smile as he gladly allowed his glass to tap against everyone else's. 

"To Sans and Frisk!"