Peter had fun yesterday and they had parted with the promise to do something like this again soon. Stiles still had enough on his list to keep them busy for weeks and Derek had said something about dinner once or twice a week and maybe a movie night or board games? The last bit had come with the warning that Stiles was a killer at Trivial Pursuit to which Peter had mentioned that he had a broad spectrum of knowledge as well which Stiles had taken as the challenge it had been meant to be.

After what Derek had told him about the open relationship they were having, Peter had been worried that things would be awkward but they had just a fun day. Peter had chatted equally with both of them and it hadn't felt forced in the slightest. He and Derek had rolled their eyes at Stiles' antics but only minutes later he and Stiles had teamed up to get Derek to try the chili hotdog.

It had been easy and it had been fun and not once had the sexual tension between him and Stiles stood between them. Peter had caught himself looking at Stiles' butt when he'd been ahead but seeing him eating a hotdog had not been as sexy as Peter had feared. However, those moments had been short and far in between, most of the time Peter had just enjoyed the company.

Stiles and Derek had normal jobs they had to return to on Monday which gave Peter time to take care of a few things. He made some calls and wrote quite a few messages, letting people know that he was in town and that he intended to stay for a while. By noon he had an invitation to dinner with Deucalion the next day.

Peter was just wondering if he should leave his room to grab some lunch after he'd been cooped up in here the whole morning when his phone buzzed with a new message. Expecting another one of his contacts messaging him back Peter reached for his phone but the display read Stranger.

Are you free later today?

Peter stared blankly at the message. A few days ago this would have been a clear booty call but things had changed since then.

Stiles must have realized how his message sounded because a second later he added:

Don't worry, I don't want to jump your bones

Well, I do. But not today

But we should talk about jumping bones

Peter still wasn't sure if this was a good idea, most likely not, but he agreed to come over in the afternoon.

The fact that Derek wasn't home when Peter arrived at their place made it even clearer that this was not a good idea.

"Coffee?" Stiles offered when they sat down at the kitchen table.

Peter agreed to the coffee and then he waited for Stiles to start talking.

"Okay, we're adults," Stiles said. "Let's handle this like adults."

"I'm listening," Peter said but couldn't help but notice Stiles' long fingers fiddling with the cup. He remembered those wrapped around his dick and he would be lying if he said that he didn't want that again.

"I liked what we did," Stiles started. "I'd like to continue with that."

"We shouldn't." Peter shook his head. Derek had said that it would be fine by him, even that he would prefer it to be Peter, but this could only end badly.

"But you would like to," Stiles pointed out. "I know that Derek and I don't have a traditional relationship but it works for us. It's just like asking a friend to watch a movie with you when your partner isn't interested."

"It's a bit different than that."

"Is it?" Stiles asked. "Sex is just something people do. It's an activity like playing baseball or watching a movie. Derek is not interested in it so I find people who are. It doesn't mean that I don't love Derek. I do. I want to grow old with him. And he knows it. He trusts me."

"I don't doubt that." Peter watched with amusement how Stiles' gestures became more exaggerated. "You made clear what you want and where the line is right from the beginning."

"So where's the problem?" Stiles asked. "Is it because you and Derek are related? That's what bothered Derek the most. I think it's the images in his head. I for sure don't want to think about somebody from my family having sex with Derek. I would need brain bleach for weeks." Stiles shuddered at that thought but then he continued: "That's why I told him that I wouldn't see you for sex ever again. Would be a shame but if Derek isn't comfortable with it I won't do it. Didn't expect him to tell me that it would be okay two days later."

"He told me the same." Peter nodded. "Says that this way it would be safer for you."

"Yeah, no more stranger danger." Stiles cracked a smile. "At least for the time you stick around."

It was tempting, Peter couldn't deny that. But things would get muddled rather quickly and this would become a Hollywood love triangle where the woman in the middle, or in this case Stiles, had to choose between the nice guy and the bad boy. In Peter's opinion, the woman in the movie would usually be better off with neither of them and the movie should end with at least one of the guys in prison for stalking but since when did Hollywood care about realism.

However, this was reality and Peter didn't see a happy ending on the horizon for them. Well, not the Hollywood kind of happy ending. He caught himself watching Stiles' mouth when he took a sip from his coffee, still waiting for an answer.

"I don't want to be the reason you and Derek break up." This was the most honest thing Peter had said in quite a while. With anybody else as Stiles' husband, he couldn't care less but this was Derek. He was the only family he'd left. After all that Derek had been through it was a miracle that he was having a normal life. He was happily married. Peter didn't want to take that away from him for a quick fuck.

Stiles studied him for a long moment.

"Same rules as before," Stiles finally said. "We're strangers with benefits, maybe updated to friends with benefits but that's it. We meet in your hotel room but what happens in that room stays in that room. Outside of that, you're Derek's uncle and we do family stuff. No sweet talking, no PDA, nothing. If you cross that line it's over. No I didn't mean it like that or I'm just kidding." Stiles fixed him with a firm look. "If you pull bullshit like that we're done. No second chances."

"You know what you want, I like that in a man." Peter raised his cup in a toast. "What if you cross that lines?"

"I won't," Stiles said matter-of-factly. "I love Derek. If I do sweet talking or use pet names it's directed at him. If you see me holding hands with somebody, it's with him. Don't you dare and try to get between us, you will regret it."

Stiles meant it. The other day Peter had been thinking about murdering him in a dark alley, however, Stiles' look and posture right now told him that he shouldn't be so sure about who would be coming out of that alley alive.

"Can I think about it?" Peter asked because damn, seeing this side of Stiles made him horny. He shifted in his seat, he suddenly needed a bit more room between his legs.

"Sure." Stiles gave him a knowing smile. "I try to stick to Saturdays with this anyway. No point in sitting at home drooping while Derek is out half the night, chasing stars."

"Derek phrased it the other way around."

"Sure." Stiles drew out the word. "Do you have any idea how many nights I sat alone at home while he was having fun, freezing his ass off out on some windy field just because Jupiter was doing some fancy stuff? But yeah, he's doing me a favor." Stiles huffed but in good nature. Either way, it did help to lighten the mood.

"Just think about it," Stiles said. "If you say no, that's totally cool. We'll just stick to the family stuff and pretend that we've never seen each other naked."

"As if you could forget about seeing me naked," Peter joked.

"Overconfident much?" Stiles countered but the way he licked his lips betrayed him. "Anyway, Derek should be home soon. Want to stay for dinner?"

Derek was the cook in the family but Stiles knew his way around the kitchen as well. Enough for some basics, as he put it. So when Derek came home half an hour later, dinner was almost ready. Just a simple pasta dish with tomato sauce and topped with lots of Parmesan. To make it somewhat healthy Stiles had thrown together a quick salad to go with it.

Peter had offered his help but there wasn't much to help with so he'd mostly watched from his place at the table. With that one topic out of the way, they talked about other things and to Peter's surprise, they had quite a few interests in common. When Derek came in they were talking about Greek philosophers.

"Hey," Stiles greeted him as if nothing out of the ordinary was going on. "Dinner in five."

"Sounds good." Derek went straight for his husband and slung an arm around his hip. Peter expected it to be a possessive gesture but it looked more like something he was so used to that he didn't even notice it any longer. He drew Stiles close but more to get a chance to peek into the pot than anything else it seemed.

"I hope you're fine with taking home some leftovers." Derek turned to Peter. "Stiles never gets the portions right and I'm not going to eat pasta for the rest of the week."

"This would never last us a week," Stiles protested. "Three days tops, with Peter here two, especially if he takes some with him."

"If you say so." Derek didn't sound convinced.

Peter watched their interactions with interest. They did banter like an old married couple but what impressed him the most was how casually Derek acted around him. He knew what he and Stiles had been talking about earlier and still Derek didn't try to intimidate him or set up rules of his own. Looked as if he'd meant it when he'd said that this was something between Peter and Stiles.

Over dinner, Stiles brought them back to the old Greeks. They hadn't just been philosophers but mathematics and astrologists as well, topics both Stiles and Derek knew quite a bit about. It didn't take long and Derek mentioned Eratosthenes who was famous for proving that the earth was round.

"He pretty accurately calculated how big it is," Stiles added. "I love to bring him up whenever a flat-earther wanders into my class."

"Don't get me started." Derek grimaced.

"That bad?" Peter guessed.

"Worse," Stiles said around a mouthful of pasta. "A few years ago there were one or two but nowadays I waste at least an hour at the beginning of the semester, telling them that math doesn't lie and if they can't get over it, they better don't come back."

"Do they come back?" Peter wondered. Listening to them he was glad that he'd not become a teacher. There would have been bloodshed.

"Some don't. But those are the ones I can live without." Stiles finished his plate and was eyeing the bowl, pondering if he should go for seconds. Peter hurried to indicate that he was full before Stiles could load his plate as well.

"I don't see that many but when I get one, it's a hardcore believer," Derek spoke up. "How can one think that astrophysics is the way to go for them if they firmly believe that the Earth is a disc?"

"I could get behind it if they added the turtle and the four elephants but when I mention those they look at me as if I'm the insane one." Stiles filled his plate but that didn't make a dent in what was still left in the bowl.

"Nobody knows the classics today," Peter agreed and leaned back in his seat. He could get used to this.