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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If picking out her dress hadn't been her favourite part when it came to preparing for this wedding, then as another choice as what her favourite thing had been in all of this so far, was picking out the outfits in which Luna and Leo would be wearing, being a cute little bridesmaid, and brides boy. Frisk found it difficult not to pull the edges of her lips into a wide smile, clearly showing to anyone in a couple foot radius that she was practically glowing with joy. Her beaming grin was so contagious to the fact that her fiance found it hard not to smile alongside her, (even though he was already smiling by quite a great amount anyway), as they were watching Leo and Luna prance around the shoe shop they were currently in, trying on different pairs of shoes that would go with their chosen outfits.

"I can't wait to dress like a pwincess!" Luna squealed excitedly. "And mommy will get to be a pwincess too!" Both parents chuckled at the 6 year old's enthusiasm, while Leo was still silently testing as to whether or not the shoes he was currently trying were comfortable. Sans glanced towards him.

"so, how do they feel? comfortable?" Sans questioned, both towards Leo and his sister. Luna ended up nodding enthusiastically with glee, while the younger brother gave his best grin and nodded himself, just in a more controlled manor, when compared to his sister. "that's good. guess we'll get these then, eh?" Sans said, also going to look back at Frisk, as if to silently ask for her opinion. She nodded. 

Both children removed their shoes, to hand them to their mom and dad, so that they could pay for them at the counter. Frisk decided she would volunteer to take them to the desk, while Sans could help their children with putting their normal shoes back on. Of course, he gladly agreed, even if sometimes it could be a bit of a fuss to make sure both 6 year olds actually put their shoes on the correct feet. 

Placing the shoes on the counter, the Dreemurr girl did her best to listen to the saleswoman behind the desk, while also deciding to space out into her gleeful thoughts, imagining how utterly adorable their son and daughter were going to look in their outfits. For Leo, they had decided to get a small suit that fitted him perfectly. It consisted of grey, with a black tie, and of course a white shirt, and he looked so freakishly cute in it. Luna on the other hand had a white dress, that stopped just above her ankles, and she would even be wearing a white flower crown to match. Being able to see her two kids who she was so proud of standing near them, looking so sweet and smart, really made her heart feel all warm and fuzzy.

Luckily, she managed to snap out of her daydream, just as the employee standing before her told her the exact amount that the shoes would cost. Having the money ready within her hand, she handed it over, only to wait patiently for the change that would be returned, with the two pairs of shoes that were also handed back in a shopping plastic bag. "Thank you." Frisk said kindly, turning around to find Sans, Leo and Luna waiting by the automatic doors for her. Sans was keeping a hold of Leo and Luna's hands, as they exited the store together, to be able to start heading home. 

"someone looks happy." Sans joked, grinning towards Frisk, who turned her head in his direction to beam back towards him. He knew she was very likely excited for the wedding. He was too, surprisingly-- only because when he first thought of the idea of proposing to Frisk, he had been so, so, so damn nervous, it was unreal. Like getting married might have just been too much, like it might just end up changing everything. Divorces were also a big fear. 

Was his fear of a possible divorce increased after his and Frisk's break up? Actually, not really. In fact, it made him glad it happened then, and that they were able to fix it, rather than when they were actually married. True, they were bound to have future fall outs, and arguments, because no relationship was perfect, and you were only naive to think that any relationship should be argument free. Sometimes disagreements were unavoidable, but what was important was to be able to talk them out, to realise the errors, to be mature about it... 

Frisk definitely had changed him. A lot. He knew he could still be cranky sometimes, but he thought of himself as a much better person, compared to how he turned out before he had met Frisk. As a child, he had been fairly relaxed. He had been perhaps somewhat hopeful for the future too. But then everything changed, and he became a person who absolutely no one could stand to even spend a couple of minutes with. How Asriel even managed to sometimes go out on missions with him from time to time was beyond him. He was an asshole. Could still be an asshole, but he was a bigger asshole back then. He dared to say that Frisk made him the best he could ever be, better than what he originally had been, as a small skeleton boy. 

"Yeah... I am." Frisk nodded. "You're happy too, right?" Frisk wondered. 

"of course." Sans spoke truthfully, knowing he meant that with all of his soul. How could he not be happy with her? Being apart from her felt like a nightmare he would never be able to wake up from. During their split, even if it had seemed like, (at least to him), that Frisk was mostly in the wrong, he couldn't stop thinking about how much he just wanted to be with her anyway. How she drove him crazy because he was just so damn in love with her. He never knew it was possible to care about someone this much. Hell, he never knew it was possible for him personally to love someone this much. But he did. 

Falling in love for the second time, (after being with Blair), had been pretty scary, and foreign to him, as the idea of romance and affection just wasn't his thing after his first terrible breakup. He had never wanted to love again. He had wanted to keep people as far away as possible, to make it so it was never possible to cause him such agony that Blair had caused him. Was he still angry at that fox for it? Not really. Not anymore. He had let go of the past quite a long time ago. Was it strange that sometimes he was glad his first relationship ended, because otherwise he never would have ended up with Frisk? Somehow, he just felt like even if it had lasted longer than it did, his relationship with Blair wouldn't have lasted forever. Whether it be from lack of interest, or them just falling out, it probably wouldn't have. 

They were happier with separate people now. At one point, Sans had thought that Blair was his one and only, but he had been wrong. Frisk was. And he knew it, for sure this time. There wasn't even any doubts in his mind that this girl wouldn't always remain at his side. She had made stupid mistakes, but she was proving herself to be worthy again. Frisk wasn't a bad person, she never, ever had been.  He was just glad he had her. Glad he was still marrying her, that he didn't lose her. He was glad that she hadn't been killed alongside her parents when they had been murdered, he was relieved that she didn't starve on the streets when she was all alone, he was glad that she ran into him that day, to save some stupid asshole's life... Just... It made him feel like the luckiest person in the world to have her, and it was difficult to put into words how far his love stretched for her. 

Some may not have thought of him as this lovey dovey guy, who cared so much about his relationship. As sometimes, people viewed guys as people who were just there for the ride. They didn't care about the love, the romance, having a future together, being together forever... But he did. He really did. He knew, being a monster, there was always the chance that he might outlive Frisk, but that was okay. That was perfectly fine. For however long, he wanted to stay with her. 

He'd stay by her side, always.