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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At first, when that party had started, Chara had been extremely nervous about it, with whatever that skeleton and human had up their stupid sleeves. But when she had found herself, kissing the one boy who she wanted to kiss, because she just couldn't deny how she felt... It had been amazing. At first, Chara had been angry towards the two for enacting out their ridiculous plan, but when it actually got her realising how Asriel actually cared about her, she was extremely thankful towards the couple for wanting to help her. The rest of the party went off without a hitch, and it felt really nice, to be able to just stay close to Asriel, to now be able to hug him from the side in this whole new way, while watching everyone enjoying themselves. 

When the party was over however, and Asriel and Chara were left alone, it felt like a whole different story.

It was almost like when the last of the guests had left, the goat and human weren't quite sure what to say to each other. No, not in that kind of bad, everything was super awkward sort of way. Not at all. In fact, when the two glanced at each other now, they found themselves stupidly smiling at each other, in that shy kind of "I love you" sort of way, but because it was so new to them, to have their feelings out in the open, it was difficult to choose what to talk about now that they were basically... Boyfriend and girlfriend. That was definitely a shock to the both of them.

"I'm glad I was finally able to tell you." Asriel eventually sighed, deciding to put his arms around Chara in a caring hug, which Chara knew was going to take some getting used to - not that that was a bad thing. Matter of fact, the shorter girl found herself smiling like a complete idiot, like a schoolgirl with a crush. It was a position she never saw herself being in, especially because Chara was more of a tough tomboy, than she was a girly kind of girl. But here she was, feeling stupidly giddy when the goat pulled her into his arms. "You have no idea how long I've wanted to tell you..."

"Yeah?" Chara chuckled, looking up at him, as her hands were resting upon his chest from the way that he hugged her, so it had left her arms crumpled up in front of her, not that she really cared. "How long? And if you say longer than a couple of months, I'll have to kill you." Chara said jokingly, but then she saw the slightly guilty look on Asriel's face and she could only raise her eyebrows at him. She had been joking, but it was longer than that? "How long?" She questioned. 

"Is... A couple of years considered as too long?" Asriel tried, smiling guiltily, while Chara looked like she was ready to whine at him in frustration for such a thing. Years?! How could he have just left it for so long and not say anything?! Not even for her benefit, but just himself. It sucked, keeping such feelings to yourself. No matter how much Chara was telling herself that she would be fine with them just being friends, she couldn't ever get those stupid butterflies out of her stomach. She couldn't get rid of that pain she felt, in her heart, where she knew she wanted to be with him. Together, as more than friends. It wasn't easy, pretending you felt one way, when you actually felt another. 

"There were many occasions I wanted to tell you!" Asriel added. "But... I could never find the time. Or the guts." Asriel sadly grinned. "I was always more scared of losing you altogether if I told you that... Every time I even thought about telling you, I only chickened out." Asriel confessed. There had been many times where he had planned exactly what he was going to say, only to not be able to go through with what he wanted to say. And then he'd only want to kick himself every time for backing out; he knew every opportunity he wasted, was an opportunity for someone else to come along and sweep Chara up off of her feet. He was lucky that no one else did come along, or that she didn't end up becoming interested in someone else. If she had... That would have been too hard. After having feelings for so long, he wouldn't be sure he would be able to take such a thing.

"I guess I can understand that... But years?" Chara sighed. "It just makes me feel bad for you. I could hardly handle a couple of months, let alone years." Chara told him, shaking her head at the thought of that. Years... That was a crazy amount of time to wait around for someone, to keep crushing on someone, no matter how much they didn't seem to show interest in return, especially with all her remarks of how she could never see herself getting married, or even having a boyfriend. Such comments like that must have been so difficult for him to listen to; even so, he still put up with her, and listened to the bullshit that came from her mouth, without even a hint of a reaction. He pulled that off, for years. That was crazy.

"The wait was worth it." Asriel stated, beaming down at Chara who once again looked at him with an expression that held shock. Her? Worth the wait of a couple of years? Really? All she was, was a tomboy girl. You could literally find girls like that everywhere. Yeah, maybe it would take awhile to build up the kind of relationship and trust he had with her with someone else, but... She didn't know if she'd say she was that worth it. "You've been there for me a lot, Chara. Over the years. Even if we weren't a couple, I was just happy to be around you. You've always made me happy, despite my feelings." The goat went on. "And I would say this was good timing, because now we get to spend this year's Christmas as a couple." He pointed out.

Oh yeah, that was right. It would be the first Christmas where she and Asriel would be boyfriend and girlfriend. She chuckled, finding it hard not to smile at such a thing herself. "I can already tell you're going to be super cheesy." Chara said, and as she once again stepped up on her tiptoes to kiss the goat, she heard him say something along the lines of: she was worth all the cheese, right before kissing him. 

He could be such a silly goat... But Chara wouldn't deny that he was the best thing to ever happen to her in awhile. If she could end up with anyone, she was glad it was him. She trusted him, she cared about him, she obviously loved him. She couldn't connect to anyone else in the same way she could connect to Asriel, like they were on their own unique level, away from everyone else. That's what was nice about the relationship they shared though, to Chara it was one of those unique relationships she could never have with another person. It's why they had always been so inseparable. That's how it had always been. Her and Asriel. 

Chara may have stated in the past that she could never see herself getting married. That was because, other than Asriel, she had never met another guy who she could ever see herself spending forever with. Not another single guy had ever made her think: yeah, I could see myself with you. No one except Asriel. It was too soon to think about stuff like that, but of course how could even a tomboy girl like Chara not hope for things to turn out like that? She knew she never wanted to be separated from Asriel, even when they had just seen each other as best friends, she dreaded the day that he would find himself a girlfriend, and he would go off on his own path, to carry on his life. She never expected that girl to end up being her. Never, did she once think before feeling this way that her and Asriel become romantically attached to each other. But was she complaining? No, of course she wasn't.

It really was perfect in time for Christmas.