After the two busty beauties dried off and relaxed their aching erogenous zones, Rias guided Akeno into a gym-like room. It was matted and had plenty of space for the two rivals to fuck each other to oblivion for their boyfriend's entertainment. Akeno looked around while Rias left the room for a bit and came back with strapon dildos. A red one for Rias and a black one for Akeno. Like she said, they matched their hair colors. The two voluptuous babes just smiled at each other without saying a word. As if they read each other's minds, they played rock paper scissors to decide who gets fucked doggy style first. The first few rounds were ties but eventually, Akeno won with paper over rock.

"Okay, so let me make sure I understand the rules." Akeno said. "I get to keep fucking you with a strapon while a timer goes and the round ends when you tap out. Whoever has the longer time wins. Is that right?"

"You got it, babe." Rias acknowledged. "But we don't just have to fuck each other. Kissing, nipple play, hair pulling, ass slapping, etc. are all allowed. Sexfighting is a battle of not just power but endurance. You have to fuck hard and be able to get fucked just as hard. That's what makes sexfighting such an amazing experience."

"Couldn't have said it better myself, hun." Akeno said. "Now bend over and let me ravage that pussy!"

Rias cooed at Akeno's boldness and obeyed. She bent over, shaking her plump ass at her rival in order to tease her while Akeno attached the strapon to her crotch. Rias held still while Akeno grabbed her hips and without warning, she penetrated her best friend's pussy, making her yelp. She was slow and teasing at first, carefully pulling in and out, making Rias moan, giving her chills. Akeno reached down with both hands, one kneading Rias's breasts, the other pulling her long, luscious red hair as she increased her speed. Rias was in heaven. She was hoping to be in this same position with Issei on the night of their anniversary had Akeno not interfered. Akeno once again increased her speed, now fucking Rias as fast as she could, shoving her fingers in the redheads mouth. She spanked Rias with her other hand, hearing her rival moan as her tongue wrestled with Akeno's fingers.

Akeno then bent down, pressing her massive tatas on Rias's back, her red hair tickling her nipples as she continued to penetrate her rival. She removed her fingers from Rias's mouth and replaced them with her tongue. The two rivals passionately made out while Akeno slipped the rubber cock in and out of her pussy rapidly. Rias would've screamed in orgasm many times by now had Akeno not kept occupying her mouth. Akeno pinched her nipples, yanked her hair and slapped her ass while not slowing down in the slightest. She was like a humping machine. Rias hoped Issei was enjoying the show. He always loved watching her dominate and get dominated by other women. She was sure he wished he was in Akeno's place right about now too.

After what felt like hours of being fucked, Rias finally tapped out and the two girls separated, trying to catch their breath. As they regained their energy, the each shot the other naughty grins before in the blink of an eye, Rias put on her red strapon, bent Akeno over and began fucking her in the exact same way. Akeno squealed in shock and then in pleasure as she was given a taste of her own medicine. Rias fucked her as hard and deep as she could and unlike Akeno, didn't even bother with teasing or slowness. She wrapped her arm around Akeno's extremely long hair and yanked, again making her squeal. Rias rapidly spanked Akeno as she fucked her rival into submission. Aleno was at a complete loss. She had no idea Rias had this in her. Just as she thought Rias couldn't possibly dominate her anymore, Rias wrapped her arms around Akeno's chest, just under her tiddies and pulled her up, ramming her tongue into Akeno's mouth as she now fucked her upright. The two beauties tongue wrestled as Rias continued to penetrate Akeno. Her speed was so rapid that Akeno couldn't even orgasm without her rival slowing down. Eventually, Akeno could take no more and finally tapped out.

Once again, the busty babes pulled apart, breathing heavily as they rested their ripe, swollen pussies. As they rested, they both noticed something. Neither of them had remembered to set the timer. They laughed at each other for making such a silly mistake, embraced and lovingly kissed. Despite their rivalry and their insane sexual competition that was nowhere near over yet, they were still best friends, still loved each other and were willing to share everything, including their boyfriend and each other. They stopped making out and stared into their rival's eyes longingly.

"So, what now?" Akeno asked. "Do we call it a tie? Or do we do it again with the timers?"

"I have a better idea." Rias said seductively. "Wait right here, babe." She kissed Akeno and then got up and left.

Akeno laid down in wait, touching herself until Rias came back, holding a double-ended dildo, one side being red, the other being black.

"Forget the timers." Rias said. "We're gonna penetrate each other with this until one of us either gives up or passes out. I fuck you with the red end, you fuck me with the black end."

"I'm game." Akeno agreed. "Now c'mere and let's fuck!"

They stood in front of each other and positioned the bicolored dildo in front of their pussies before Rias gently inserted the black side inside of her while Akeno did the same with the red side. The two beauties cooed as the giant dildo penetrated them. They each got a good feel of their end before getting comfortable and giving each other bedroom eyes that Issei usually has to do something amazing in order to have them shot his way. They humped into each other, making them both squeal. The dildo was enormous but both girls were able to handle it getting pushed inside of them so much that they were now face to face and tit to tit.

"Damn..." Akeno said. "How loose does your pussy have to be to handle something like this?"

"Look who's talking." Rias shot back. "I can stick my entire fist inside of you, slut."

"Bitch!"

"Cunt!"

Akeno then pulled Rias by her hair into her lips, sloppily tongue-kissing her, which Rias fiercely reciprocated as they continued to hump each other with the dildo. Rias yanked Akeno's hair in turn as they kept this up for what they didn't know was hours and hours. They had repeatedly squirted and sprayed each other with their pussy juice as they relentlessly tongue-wrestled, neither of them giving up or showing any signs of exhaustion. They just kissed and fucked and kissed and fucked and kissed and fucked long into the night, only briefly pausing to insult each other, pull hair, spank asses, knead breasts, etc.

They had kept going with this repetitive rhythm for six hours until finally, the limitations of their bodies caught up with them and they both fell to the side, barely enough energy to move as they moaned and panted. They looked up at the clock, realizing that it was now past midnight. They'd been kissing and fucking each other for a quarter of a day. Once again, they smiled and laughed at how carried away they got by their competitive spirit.

"I hope Issei realizes how lucky he is." Akeno said. "Having babes like us for girlfriends that are willing to fuck each other into oblivion for his viewing pleasure."

"You know he loves and respects us." Rias replied teasingly. "And this just proves how much we love him in turn." Akeno rolled towards her and hugged her lovingly.

"C'mon, babe." Akeno said. "This little contest of our's is just as much for us as it is for him." Akeno kissed her.

"Quite true." Rias admitted. "Now let's go to bed and get some rest. This was just the first day of a long week that will put our womanhood to the ultimate test."

"What about this?" Akeno asked. "Do we call it a tie?"

"Sure." Rias agreed. "But we have a long week ahead of us. I'm still gonna fuck you like you've never been fucked before."

"I was gonna say the same thing, skank!" Akeno teased. They playfully wrestled and kissed for a bit before getting up and leaving.

They got into bed, cuddling each other while making out until they finally drifted off into sleep, excited for the erotic competition that the next day would bring.