Sans is a MONSTER and not a HUMAN, so he doesn't follow HUMAN biology like a HUMAN skeleton. Same with Gaster and Papyrus.   

Sorry that this part is quite short.

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Sighing, the skeleton placed the piece of paper he was looking over upon his desk within his lab, as he spun around in his chair slightly, putting the back of his chair to his desk. For the past couple of days, Gaster had been thinking a lot on his old agreement that he had made to Asgore, in the first couple of years that he had been a part of the Mafia. Originally, he had accepted the thought of possibly becoming the next Mafia boss, if anything were to ever happen to the goat man, or if he were to step down. Ever since then, things had changed so much, that he hadn't been sure he would be confident enough to have such a job. Even now, he questioned if he would even be capable.

Gaster may have seemed like a fairly confident man; it made it easier to keep up such a facade when he was used to putting on a poker face. Not to say that he was never confident, but when it came to such a huge responsibility, it did cause slight panic to seize his bones. However, the reason he had been thinking more about it as of late, was because he was thinking about his son - Sans, and his relationship with Frisk. The other day, he had come back to his house, seeming extremely depressed. It made it worse when he didn't have Leo at his side, or Luna. The skeleton was clearly less worried about keeping a mask on around adults, than he was around his children.

Not only was Sans clearly down in the dumps about everything, but so was Frisk. She wasn't as positive and bubbly as she normally was, and that was very clear, even when she tried to play pretend. She was still thinking about the fact that her and Sans' relationship was over, and that was obvious. Picking himself out of his seat, he decided he would take a short break to visit Grillby. He wanted to talk about what was on his mind, and whether Grillby agreed whether or not it was his responsibility to do something. He couldn't stand just standing by, while watching his son clearly fall apart over this whole situation.

"Hey, Grillby." Gaster smiled towards his fiance who was standing behind the bar, after stepping out of the elevator, and the flame monster gave a small grin in return, seeming glad to see him. To the skeletons surprise, two faces were also sitting at the bar - Blair and Andrew. "Oh, hey you two. It's been awhile." Gaster stated, going to sit down beside the couple. After all, Blair and Andrew themselves weren't here as often; they seemed like they were much busier getting on with their lives, especially since living together. 

"Yeah, it has." Blair nodded. "I heard about Sans and Frisk. What's going on with those two?" The fox questioned in bewilderment, causing Grillby and Gaster to exhale, as it felt like a terrible chore to explain, only because it was an unhappy subject. Even Andrew was frowning, worried about the happiness of his good friend. "I just don't understand why Frisk doesn't step down from being the Mafia boss. It's clear she's unhappy doing it. Relationships are more important, I think." Blair stated, only to smile towards her boyfriend who she lightly kissed on the cheek. 

"It's more complicated than that." Gaster said. "She doesn't want to quit because if she did then... Who would take her place? Everyone would lose their job." Gaster explained, frowning towards his own hands which were pressed together on the bar in thought. "Which got me thinking..." He started, glancing towards Grillby. "The only person who can really take her place is me." He said, seeing a look of confusion spread across Grillby's face.

"Wait, why you?"

"A long time ago, Asgore asked me if I wanted to be the next boss. At the time, I had accepted." The skeleton clarified. "But then I lost my wife, and so much changed since then... I suppose I had changed my mind." Gaster shrugged. "Frisk will only step down if someone else stands in for her, so..." 

"But aren't you two getting married?" Andrew wondered. "What makes you think that it's any better of an idea for you to do the job than Frisk? You still have a relationship to keep intact." The human went on. If Frisk couldn't do it, without destroying her own relationship, and the same with Asgore, what made Gaster think that he could handle both his time at work and his normal, every day life? 

"Well... He's already a bit of a workaholic..." Grillby said, rolling his eyes a little, but he was smiling towards Gaster, which showed that he wasn't really angry. In fact, he and Gaster did spend a lot of their time working already. And neither of them really minded, especially because they would see each other at work a lot. Gaster would make the effort to get out of his lab, to come up to see him. Sometimes Grillby would even make sure to bring down food for him, in case he forgot to eat. He knew how engrossed Gaster could become in his work. 

"So, do you think I should do it?" Gaster wondered. Grillby didn't even hesitate to nod.

"I think, if you think you're capable, and you want to, then you should... Don't just do it because you feel like you have to. Because that will put a lot of unneeded stress upon you." Grillby said, and the couple sitting at the bar next to the skeleton could only nod in agreement. "Though, I was wondering, if it's possible you can wait to let Frisk know you'd be happy to replace her, at least until after we get married." The flame monster beamed. "Then we can get right back to work." 

"You two are so sweet..." Blair stated. "Literally couple goals." The fox joked, causing the flame monster and even the skeleton to look elsewhere, seeming embarrassed as the two of them softly blushed. Even when they were almost a married couple, sometimes they acted like a pair of teenagers who didn't know how to handle people commenting on their relationship. Especially because at first, they had always been best friends, and it had been so crazy to have gone from that to being a couple. Neither of them regretted though. They were extremely happy, being with each other, and Gaster knew he was happier, being with Grillby.

Even if romance hadn't been Gaster's main concern before him and Grillby got together, after losing his wife, even years later, things had felt incredibly lonely. Even with friends, and people to care about him, he just knew that he would keep feeling lonely, when he found himself sleeping all alone at night, feeling like he would probably be alone forever. Getting together with his best friend, had changed his life so much, definitely for the better. 

Their wedding wasn't actually that long away, and the couple knew they were growing increasingly both excited and nervous for it. Because people were always nervous on their wedding day, because it was just inevitable you were going to feel that way. Just because Gaster had been married once in the past, and had already experienced a wedding, but even now, he knew he was going to be extremely nervous when that day came. 

It would be the best day ever.