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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"Still no ring on your finger, are you kidding me?" Chara complained, looking at both of Frisk's hands, as they were sitting on a park bench together, with Sans busy giving Luna a push on a park swing, and Leo was running around kicking a football around. Frisk sheepishly smiled, pulling her hands away from her sister's, who sighed in exasperation. "I swear... Sometimes I question if that skeleton was the best person for you." She said, seeing the look that Frisk gave her. "I'm just joking! I just wish he wouldn't take forever to ask you about such a thing." 

"Well, we have busy lives... Maybe he just hasn't had the chance to." Frisk shrugged. 

"Why do you have to wait for him to ask you? Why don't you just ask him?" Chara asked, knowing at this rate if Frisk kept waiting for Sans to ask the question, he was probably never going to ask it. It probably wasn't even entirely the idea that he didn't want to get married. Knowing him, he was too scared to ask, or it was too much of a big deal to him to even grapple the courage. "I know it's expected that the guy should always propose, but..." 

"...I don't know. I've always had this nice idea of him taking me on this romantic date, and then he would just ask." But these days they didn't have as much time to go on a date together. Maybe that was partly her fault, trying to carry two jobs at once when she probably only needed one. However, she wasn't really sure which one she wanted to quit. If she quit the Mafia, chances are she might not see everyone else as much, and if she quit the cafe, she would no longer see Kai and she felt like it would just be too much. 

"Have you ever told him you want to get married one day?" Chara pondered, knowing the answer was most likely no, because she wouldn't want to frighten the skeleton away, and it was just as she guessed. Chara didn't think that even now, years later, she would still be practically acting as a cupid for their relationship. She exhaled and decided to get up from the bench. "If you really need help that badly, then allow me." The sister volunteered, already going to go over to Sans. This gave her a lot of Deja vu back to moments when Sans had been too frightened to approach Frisk. These two were too cute sometimes, even almost 7 years later.

"Hey, Luna? May aunt Chara get to go on the swing? I need to talk to your dad for a moment." Chara beamed brightly at the 6 year old, who happily nodded and got off once the skeleton had stopped pushing her. Sans just stared at the Dreemurr in bewilderment, but allowed her to sit down on the swing. He didn't know whether to push her or what, but his internal question was soon answered by her next question. "Are you going to push me or not?"

"what age are ya again? aren't ya like 24?" Sans said, but with a roll of his eyes, he did begin to give her a push on the swings, and it stayed quiet between them for a short while. However, Sans was suspicious of her. "okay, what do ya want? ya wouldn't have come over here and kicked my daughter off the swing if ya didn't want anythin'." Sans grumbled, while Chara scoffed at how he phrased that. He made her sound like she was being a bully, but Luna had happily gone to join her brother at trying to play football. For a brother and a sister, they got along pretty well, but Frisk and Sans always figured it was because they were twins. They may not have been identical, but they definitely had a special bond. 

"Well... You've been with Frisk for almost 7 years..." Chara started, but the second she said that, Sans grabbed the chains of the swing to stop her from swinging. If she hadn't also gone to grab onto those chains, she might have smacked her head on the concrete by falling backwards. When turning around on the swing to try and look at Sans, she could see the pupils were gone from his sockets. He must have already knew what she was going to ask. "What?"

"let me guess. ya were gonna ask: why haven't ya asked frisk to marry ya yet?" He answered, knowing just by the grin on her face, he had caught her red handed. First Mettaton, now Chara... Well, they weren't the only two who had bothered him about it. A lot of their friends had asked them. Even his own brother had questions. "look, it doesn't matter. marriage just makes things more complicated. it makes things... different." 

"So, let me guess. You just don't want to admit you're scared." Chara rose an eyebrow at him, causing him to growl once more. "Sans, she's going to want you to ask eventually... If you don't, what if she does get bored? I know you guys have kids and all but... You got to have at least something else going for you, otherwise..." Chara shrugged, as if to say: what's the point after that? Frisk wanted to get married, and while she struck Chara as the type of girl who would gladly wait around for Sans, she didn't think it was fair on her. "And what are you going to do if Frisk proposed to you? Say no to her?" 

"well... no, but..." Sans started. 

"Just stop overthinking everything with that big fat skull of yours. I guess you thinking too much about things is going to be something that will never change." Chara joked with a sad smile, causing the skeleton to sigh. He knew it was an annoying characteristic about himself, but he just couldn't help it. It was just permanently wired into him at this point, and he couldn't stop himself from doing it. Standing up from the swing, Chara called Luna back over and said she could have it back, which caused the 6 year old to cutely say: thank you auntie Chawa, before getting back on the swing.

"She warms my heart so much..." Chara gushed when she had sat back down on the bench next to Frisk. "He doesn't talk much though. I can already see who he is more like." Chara joked about Leo, causing Frisk to lightly smile at such a thing, but she was expectantly sitting, waiting to hear whatever her sister had said to Sans. "Yeah, you should ask him." She finished. "I think he's too scared to make the move himself, but if you ask him, he'd most likely say yes." 

"You think so...?" Frisk wondered, staring towards the skeleton with a thoughtful expression on her face. Usually she was really good when it came to reading Sans, she would usually know what he was thinking. But where he stood on marriage was a whole other story, and she could hardly tell what he wanted. Would he really say yes to her if she asked him? And if she asked him, what would she even say or do? Where would they go? There was that place they had gone to for a really nice date a few months ago. It was actually the same place where they had celebrated Frisk's 20th birthday, and they had one of the most first meaningful nights of their growing relationship. Maybe they could go there...

"Glad that I could help." Chara teased towards Frisk, already noticing how the cogs were turning inside her brain thanks to the advice she had given. 

It might only be a matter of time now.