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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Annoying customers were always the worst. Grillby had had his fair share of them, as had Frisk, considering she was still earning money working at a cafe, and it was no surprise to her that some people did choose to come in just for the sake of seeing two girls in some kind of uniform. It had taken everything in Sans not to punch a couple of people in the face when he couldn't help but notice random guys drooling over his girlfriend. That was one of the negative parts about not staying at home, he had to witness such bullshit as that, and as much as he wanted to knock anyone out for it, he couldn't do a lot as he didn't want to cause trouble. Though, there was one day he found it difficult to stay by that rule. 

It was like a lot of other work days, where Frisk had come into work with her boyfriend who just decided to sit at a table, waiting for her shift to be over. He knew that ever since this whole pregnancy thing had started, he had become extremely clingy. But he just hated to be left alone at home with no one to talk to. Well, okay, true, maybe he was never completely alone anymore, but having no one else around made him extremely anxious. He was always fearing that something bad might happen when no one else was around and he wouldn't be able to do anything about it. 

While he was thinking to himself about the amount of time he had left until all of this was over, his pupils couldn't help but trail up from the table he was sitting at when he noticed three other guys appeared to stroll into the cafe, looking like maybe they were in their mid twenties at least. Sans couldn't help but judge the moment they walked in, as the three of them were looking somewhat scruffy, and each of them had different shades of hair - one had black hair, one was blonde and then there was one guy with dark brown hair. But maybe that was just him being judgmental about three people he had never met before. The three guys went to their own table, while Frisk had gone to go over to their table, likely to take their order. Sans sat, watching closely with those two pupils of his. 

"Hello, may I take your order?" Frisk asked kindly to the three strangers, while Sans continued to watch and listen. It was difficult not to notice the three guys seeming to look Frisk up and down, and then they ended up grinning at each other. The skeleton narrowed his eyes in frustration, feeling a spike of jealousy. Not because he didn't trust Frisk, but just because he didn't trust them, and he felt it was probably for a good reason. 

"Wow, she's cute." The brown haired one said to the two others, who just nodded and chuckled in agreement. Frisk had received such comments so many times that any time she would be complimented now, she would just happily smile and say: thank you, just to humour them and avoid a silly fight. So, after saying that, she asked them once more what they wanted to order, and the brown haired man continued. "Is it possible to order you to sit with us for a bit? Because I didn't know there were cuties on the menu." 

Don't punch anyone in the face, don't punch anyone in the face... Sans had to just think that over and over, knowing that Frisk likely had the situation professionally handled. Matter of fact, even now she was somehow smiling, despite being uncomfortably flirted with by three other guys. To tell the truth, Sans would have expected her to look uneasy in such a situation, like that time when she had been ambushed by a couple of strangers in the street, and she cried out for help and he came to her rescue. But right now, she looked quite calm. Had she really matured that much since then? 

"That's very funny. But sir, I'm afraid that if you aren't actually going to order anything, then I'm going to have to ask you to leave." Frisk said, keeping it professional. When they didn't seem like they were taking her seriously, that is when she decided that these guys really were here just to fuss over her because she was a "pretty girl in a maid's outfit". Noticing Sans staring their way, Frisk gave the skeleton a kind smile, showing him that she was okay. Because she was, these guys didn't exactly frighten her. After everything she had gone through with being a part of the Mafia, she wasn't sure if people could phase her that much anymore. 

With that being done and said, Frisk was ready to turn away and leave, at least until she was stopped as a hand grabbed her wrist. "Hey, wait! Come on now, you aren't being fun." The blonde guy grinned, using his grip on her wrist to pull her closer as he had stood up from his seat. It was then that Sans got up from his own seat, looking ready to pound someone. Upon him getting up, that's when he was noticed by the three guys. 

"Tch, what are you looking at? Mind your own damn business, freak." The black haired male said, only for Sans to growl. The fuck was this dipshit on about, mind his own business? He was literally harassing a girl with his friends in a public place, did they really not expect anyone to pay attention to what was going on? Even Kai who had come from the kitchen noticed the situation, and ran to get the managers as she was scared of what was happening. 

"how about ya get your hands off my fuckin' girlfriend? how's that for my business?" Sans spat, but saying a thing only seemed to amuse the three men even more, especially considering they were actually taller than the skeleton, so when they had Sans yelling at them about what they were doing, all they could do was snicker and laugh, like what he had said was just completely pathetic. Hell, the black haired guy even dared to go right up to Sans and give him a slight shove, and Frisk was getting so ready to argue for that. Sans definitely did not look pleased because of such a move. In fact, he looked pretty ready to bite his head off. 

"Well, she's my girlfriend now, so take that." The blonde man said, looking back at Frisk who was glaring at him in anger. The moment he even tried to kiss her, it was then that Frisk tugged her wrist from his hand and she slapped him right across the face. No one was allowed to try and make such a move on her, no one but Sans. And she hated that these three men were coming in here, trying to make a scene. 

"Excuse me, but you don't get to decide that for me." Frisk said. "Like Sans said, he is my boyfriend. So, maybe you three should go elsewhere to try and get in a girl's pants." She snarled, as these three people had taken it way too far. Normally she was used to the kind of thing of people trying to stupidly flirt with her, but if it started to get physical, and they tried to make a move on her, that was where it was completely unacceptable. When one of the other men tried to grab her wrist, she only used that to pull them to her, just so she could kick them... In the place that God made only to be treated nicely. It was probably no surprise that the guy stumbled to the floor, holding his crotch in agony after that kick.

"You three have to leave." Mina - one of their bosses - yelled as she came into the room, after being retrieved by Kai for help. "Otherwise I will be forced to call the cops." She said, knowing that that was enough to cause the two other guys to quickly grab their pained friend and make a run for it. "Don't come back here, ever!" Mina shouted after them once they were out of the door, until she turned back to the others. "Thank you for fetching me, Kai. Sorry, Frisk, if they caused you a lot of trouble... Sometimes we do get bad apples like that here..." 

"No, it's okay." Frisk giggled. "I was able to take care of myself, but thank you for getting rid of them, and thank you Kai for getting help." Frisk said to the two, until she glanced back at Sans who was looking at her in shock. She was almost ready to question what was up, until he was suddenly walking right up to her and hugging her tightly. "H-Hey, I'm okay. Are you alright?" Frisk wondered, only for Sans to quietly laugh.

"of course i'm fine. a little shove wasn't enough to hurt me... i was just... thinkin' about how much ya have matured, that's all..." Sans mumbled, seeing Frisk blush a little at his comment. "i remembered before ya were a girl who seemed terrified of everythin' bad. but ya really took care of yourself back there." He said, actually feeling quite proud of her that she didn't need him to step in all that much. He didn't really even need to do anything to make sure  that she would be okay, even though he had obviously wanted to. Protecting her was something he knew he would never want to stop doing; he also had to accept though that she could look out for herself, and she didn't always need him defending her in everything. 

"I learned from the best." Frisk teased, kissing Sans on the skull. She really had only become a tougher person because of him, and she didn't think she would be able to defend herself that much if she had never met him. Through all the tough experiences they went through together, it was only building her up into a more confident person, and confident is what she wanted to be. She didn't want to keep being that girl who cowered in fear and ran away from everything, expecting her knight in shining armor to protect her. 

She could stand up for herself too.