(I got the idea from a That 70s Show episode, somewhere here might remember it) (Also, this gets pretty hot and heavy just so you know)
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It all started because of a dumb argument. It was the fourth time this week. Twilight sat at her desk hard in their bedroom, hard at work signing official royal documents, they had to be signed right away. Although like every other day she tried so far she found herself distracted by incredibly hot Rainbow Dash who always found work as the best time to fool around. Twilight would tell her she can't she's busy, and then Rainbow would do... it. She'd softly place her hooves on her shoulders, slowly breathing on her ear, teasing her. She would then gently whisper "please.." right as she began to start nibbling on Twilight's ear. Twilight could feel her breathe get caught her throat as she felt Rainbow work her magic, slowly massaging her shoulders as she nibbled on ear gently.

In-between soft moans that Twilight couldn't avoid making the'd try to get her to stop. "R-rainbow please. This ha- has to get done" she pleaded, but she knew once Rainbow had her eyes set on something she wasn't going to let up. Though Twilight couldn't blame her for it, honestly she wasn't much better about it. After the gentle teasing Twilight couldn't take it any longer.

She shot out of her seat and turned on the grinning Rainbow who knew where this was going... because it always went there. Twilight quickly marched over to the bed as she grabbed a hold of Rainbow's tail with her magic, her purple aura surround her horn. She would quickly lift Rainbow up and toss her on the bed much to the enjoyment of the pegasus. Before Rainbow could sit up Twilight would have already pounced on her prey, pushing her back onto the bed, locking lips with that beautiful pegasus as fast she could.

Twilight laid on top of Rainbow as they kissed, both moaning in pleasure. Hungrily, Twilight sent her tongue out in search of her marefriend's tongue, looking to fight. Rainbow's tongue accepted the challenge, ready to defend it's territory. Twilight's tongue lashed forward licking Rainbow's up down and all over but Rainbow fought back. Her tongue moved quickly as well savoring every moment of it. Twilight pressed harder and more forcefully, finally subduing Rainbow's tongue with her own.

Rainbow couldn't help but see a forceful Twilight as the sexiest in the world as the normally polite well mannered Alicorn would break out of her shell to take what was hers. Rainbow instictively starting bucking her hooves upward as they kissed meeting Twilight's legs. Twilight locked onto the rhythym and started moving her hips along with Rainbow's. Twilight shifted slightly to the side so that their marehoods could meet. Twilight eagerly picked up the pace grinding her hips in tune with Rainbow's as both mares moaned out in pleasure. As Twilight humped her on top, Rainbow couldn't help but let her head droop of to the side as she panted heavily, a little bit of drool even dripping out.

"Oh Rainbow.." Twilight moaned as she continued to hump the mare below her, both reaching closer to their climax. Twilight picked up the pace grinding faster and faster as Rainbow mirrored her actions, their flanks bouncing as they did. Rainbow couldn't stop herself from sending a hoof out to grab Twilight's flanked, as Twilight back and forth quicker and quicker.

"Oh my... Twilight... don't stop.." Rainbow managed to squeak out as they both neared their climax. Their breathes quickened and the moans increased. Twilight and Rainbow were sweating from the exertion as the grinding reached full speed. It was here, it was so close, nothing else mattered to Rainbow but what she was feeling down below right now. "Here it goes... almost... there... here it............ goes..?" It came off as more of a question as Rainbow muttered. Almost all at once the pleasure had begun to leave her body as Twilight stopped and climbed off of her marefriend.

Rainbow slowly panted trying to regain her breathe as she watched in desperation as her marefriend climbed over to sit on the side of the bed facing away from her. She looked down sadly to see she was plenty wet down below, but she hadn't quite... finished.. yet, and from the looks of it Twilight hadn't either. This had to be something serious if Twilight was willing to stop that.

Rainbow slowly sat up and shuffled over to Twilight to sit beside her. She leaned forward to look Twilight in the face, she wasn't crying so that was good, but she did seem distressed. As Rainbow went to ask what was wrong Twilight answered before she could.

"Do we have too much sex Rainbow" Twilight simply asked, Rainbow's eyes shot open wide at the question.

"Too much sex? Is that even a thing?" Rainbow asked in bewilderment. Twilight just turned to her with a serious expression.

"I'm serious Rainbow, it just feels like were always having sex. We already did like four times this week, each time keeping me away from my work" she said motioning to her desk.

"D-do you... not like it?" Rainbow asked, in a self conscious tone. Twilight looked startled for a second but then she smiled a little towards Rainbow.

"No Rainbow it's not you, you're actually.. really good" Twilight said much to the joy of Rainbow, but then it turned serious again.

"I just mean, it seems like all we do is have sex, like our relationship is sort of based around that. I just think maybe we should take a break with it and focus on other things" Twilight explained.

"Wait...so you're gonna hold out on me!" Rainbow asked in fear.

"Honestly.. yeah. It's keeping us from our other responsibilities, it's irresponsible. Plus it's not like I need it to live or anything" Twilight muttered that last part.

"Whoa whoa whoa, so you're saying that I do?" Rainbow asked, Twilight nervously looked down to her hooves and back at Rainbow.

"Kind of yeah.."

"Oh I see how it is, then fine. I'm gonna hold out on you then!" Rainbow declared.

"What? You're going to hold out on me, let's be realistic Rainbow"

"Oh please, I can hold out way longer than you can. You'll be in that bed by the end of the week getting yourself some of this" Rainbow said as she motioned to her lower half. "Next i'll be on top though" she grinned.

"In your dreams Rainbow"

"Wait how do you know what's in my dreams?" Rainbow asked, Twilight just raised an eye brow.

"So are we really doing this then?" Twilight asked.

"Definitely i'm gonna out sex you, it'll be a sex off!" Rainbow declared, Twilight facehoofed.

"Alright, well then let's get one rule straight. No masturbation, at all" Twilight stated, which quickly deflated Rainbow's ego.

"No more Twilight and no more hoof?" she asked sadly, Twilight nodded at her, an evil smile on her face.

""Alright fine, it won't be long anyways like I said" Rainbow said, Twilight's cockiness hadn't subside though. Twilight reached her hoof out "So is it official?" Rainbow looked from her eyes... beautiful eyes.. back to her hoof. Her uneasiness turned to cockiness as well as she reached her hoof out and shook. "It's official, let the sex off begin!" Rainbow announced, once again making Rainbow facehoof.

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Not long after it was announced they were quick to tell their friends. They had managed to recruit two friends each to join sides with them and help them win, of course their friends had added their own wagers. Like farm work for a week and modeling for Rarity etc. The teams went like this at the news.

Twilight

"Oh wonderful darling, holding out is a lady's best weapon in a relationship" Rarity replied, happy to join Twilight's team.

"Ooooh so you're not gonna do it with Dashie, there is no way she's gonna survive" Pinkie said, in a bubbly tone.

Rarity and Pinkie Pie were set on making Twilight win.

Rainbow Dash

"Y'all really sure ya' can do it?" Applejack asked, Rainbow deadpanned at her. "Of course I can AJ, you've set it yourself I'm passionate when it comes to my goals" Rainbow replied

"Wouldn't passionate make ya' wanna bed her more? Ah hay whatever sure I'll help ya' Rainbow" Applejack relented, Rainbow pumped her hooves up in celebration.

"Um... I guess I can help Rainbow, nobody is gonna get hurt right?" Fluttershy asked, Rainbow deadpanned at her as well.

"Of course not Fluttershy. Come on Flutters were best friends, you gotta help me out" Rainbow begged, Fluttershy nervously hesitated.

*sigh*

"Okay Rainbow i'll help you"

Applejack and Fluttershy were kind of set on helping Rainbow win.