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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Frisk had wanted this date to be a surprise; she wanted it to be different to past dates they had had; sometimes simplicity was best, compared to eating out at some fancy restaurant somewhere, (not that she never enjoyed those dates where they did that, she loved every date she got to spend with Sans). She was just hoping that he would enjoy the simple little date she had planned for them, and to make sure it was more of a surprise, she went to walk behind him and cover his eyes when they were close to their destination. He had always been a little bit taller than she was, but thankfully the heels she was wearing made it easier to reach and cover those sockets of his from behind him. 

"how come i'm not allowed to look yet? ya aren't up to somethin' sneaky are ya?" Sans had teased, when Frisk was leading him along, carefully making sure he didn't trip over his own feet, (because that definitely would spoil the romantic moment). She did let out a small giggle at his silly comment, and she even rolled those ocean blue eyes of hers as she grinned at his joke. Small moments like this really made her remember why she appreciated him so much. 

"Of course not. We're basically there anyway." Frisk said, after a couple more steps, until she finally removed her hands away from his sockets. "Surprise." She beamed, showing him what appeared to be a blanket on the ground, with a picnic basket. They were currently in an empty field, which was usually used by people to take their dogs for walks or people would sometimes play football here - it wasn't that far of a walk from their neighborhood. "I know it's pretty simple, but I thought it would be nice to sit under the stars. Happy anniversary, Sans." Frisk said, going to lightly kiss the skeleton on the cheekbone, who smiled and went to wrap an arm around her shoulders to hold her close.

"i love it. happy anniversary. you're so cute sometimes, ya know that?" Sans grinned, causing her to smile before the two of them went to sit down on the little blanket that had been laid across the ground. Luckily, even though it was getting somewhat dark, it didn't feel too cold at the moment, but Frisk had been prepared if it did get cold, seeing as she did bring a dark leather jacket to wear with the rest of her black clothing, and it kept her fairly warm. "i love ya, frisk. i don't know if i say it enough, but i do." The skeleton murmured. Frisk smiled. 

"I love you too. And you do say it, quite a lot actually." Frisk joked in return, causing the monster to lightly blush at her tease. To others, he may not have seemed like it, but Sans could be extremely affectionate towards her. Sometimes they did have silly little arguments here and there, (as did most couples), but he was so sweet towards her that she was glad to have him. She wouldn't like to imagine a world where she hadn't somehow run into him that day in that alleyway, all that time ago. That day she met him, she had been pretty afraid of him. Little did she know that she had met the man she would want to marry. She could never get over how bizarre the world could be sometimes. 

The couple spent a little bit of time just admiring the night sky at each other's side, before deciding to eat some of the food from the picnic basket Frisk had prepared. She had Papyrus and Mettaton to thank for setting up her whole idea for her before she got here, as she wouldn't have really had the time to do it all by herself. She did prepare the food though, so it made her glad when the skeleton seemed like he was enjoying it. After all, with what she wanted to do, she had to have this date be perfect, and so far, it already seemed like it was going really, really well. She just had to wait for the perfect opportunity to ask what she wanted to ask, but she didn't really know when that opportunity was. Now that she was actually here, planning to ask the skeleton to marry her, she was a million times more nervous about it. Oh shit, she wasn't going to chicken out, was she? No, she couldn't after having all of this planned... It was perfect, with the stars and the moonlight, so she had to ask tonight. 

"hey, frisk?" Sans spoke up, interrupting her anxious thoughts for the time being. Okay, she just had to listen to her boyfriend and think about those panic inducing thoughts later, there was no rush to ask right now, was there? So, she looked towards him with a "yes, Sans?", only to raise her eyebrows a little at his thoughtful stare. He glanced away for a second, as his pupils appeared to randomly scan the ground, as he was clearly thinking, and then he looked back towards her. "i need to talk to ya about somethin'." 

"...About what?" Frisk asked in bewilderment, feeling a little on edge admittedly, because all of a sudden, compared to his happier mood a couple of moments ago, he was extremely serious. Not a scary kind of serious, but... It was still a little nerve wracking. She continued watching him in confusion as he went to hold onto one of her hands, intertwining his fingers with hers as he remained quiet. "What is it?" She wondered again, wanting her boyfriend to speak up about whatever was on his mind. He sighed. Uh oh, was it bad news?

"i..." He started, until he paused in hesitation. Once again, he exhaled, and looked directly at Frisk, using his two white pupils to stare right into her beautiful blue eyes. "like i said, i love ya, a lot. like... ya have no idea. heh." He chuckled a little, causing her to giggle once again with him. "ya mean a lot to me... more than i ever thought i would care about ya... the day we met, i never knew that girl who i had first seen as this annoying little pest, would end up turning into the most wonderful, and amazin' girl i had ever known... i just never knew i would feel this way, and i don't regret any minute of it." He murmured, seeming to actually blink back tears. He wanted to be able to make it through what he was trying to say without getting all emotional about how sentimental he was becoming.

"i guess what i'm tryin' to say is just that... you're the best thing that ever happened to me, alongside leo and luna. i'm glad that i got to have those two with ya, which is why i hope ya never leave me." Sans was saying, causing Frisk to feel a little emotional herself, as all that he was saying was really pulling at her heartstrings. She just continued to smile through all his words of love and affection, while blinking back tears of her own, not knowing that what he was going to say and do next would cause her to have a complete breakdown. "so i wanna ask ya somethin'. which i never thought i would ask anyone." He said, seeming to use one of his hands to reach into his suit pants pocket, and that's when Frisk's eyes widened a little. Was he about to...? 

Just as she thought that, his hand that was holding onto hers let go for just a moment, only so he could open the little box that he had in his hands, and it revealed a small little ring, with a bright blue gem. Just seeing it was enough to cause her breath to catch in her throat. "will ya marry me?" He asked, just as Frisk started to cry. She put both her hands over her mouth, not even knowing what to say. She couldn't even care that for the moment she had had the same idea. All she could do was practically tackle him into a hug, and sob loudly into his shoulder. He had let out a small sound of surprise, but he could only grin and happily hug her in return, as he could hardly keep the tears out of his own voice now. "i guess that's a yes then?" He said, his voice a little shaky from the emotions. 

"Yes, you big idiot!" Frisk squealed, going to pull away from the hug to get a look at his face. "I was wondering when the hell you were going to ask! That's what this whole date was about..." Frisk was saying, seeing a small look of confusion cross Sans' face, and that's when she went to pull out her own little box from her coat pocket, and she revealed the ring she had bought for him, with the little red gem. "But you beat me to it..." She giggled tearfully, going to try and wipe away the mascara that would inevitably be running down her cheeks from crying. 

To find that Frisk was also going to propose to him really did leave the skeleton feeling extremely dumbfounded. But he could only smile wider and begin to cry with her, as he first went to slip the ring he had bought for her onto her finger, and then she proceeded to do the same for him, until they found themselves in a really tight hug once again, as they held onto each other like they never, ever wanted to let go. "just... thank ya. thank ya for givin' me a chance to love ya." Sans whispered. Frisk laughed.

"No problem, Sans..." She whispered.