This is written in the same universe as Stuck in a Rut by RoseyRedd. I highly recommend you read her story before reading mine.

But if you don't... tl;dr: Everyone born in the past century or so has been a boy. Sometime in their teens, two boys will suddenly become attracted to each other. One will sodomize the other. The one on the receiving end becomes female, and the other is locked into male mode. This is called the rut.

So, every boy grows up knowing that he will be tested, and will come out either a boy or a girl -- and that there is no going back.

This story is marked explicit, though I can't really call it erotic. But then, I don't generally write erotica. The descriptions are there because they are necessary to the plot.

Like I said, I don't generally (ever, so far) write erotica. This isn't my usual fare. The universe intrigued me, though.

Phil and Marlie snuggled on the old mattress in the tree fort that they had built in Phil's back woods.

While the thought of using an old mattress is disgusting, Phil and Marlie, when she was Mark, had thoroughly cleaned, bleached, and sunned the queen sized mattress that Marlie's parents had tossed out when they upgraded to a king. After all, it was a shame to waste a mattress that still had lots of life left in it.

The fact that Marlie was probably conceived on that very mattress didn't even occur to the two.

The plastic sheet and several layers of mattress pad meant that the inevitable spillage from their love making never made it down to the mattress.

Early next morning, they sneaked off to their respective houses and prepared for school.

They weren't fooling their parents, but neither did their parents say anything. Marlie's contraceptive implant ensured that they wouldn't become parents before they were ready.

Phil and Marlie sat at the lunch table with their arms around each other. Since Marlie was left handed, she sat to Phil's left and ate with her left hand.

Tom came up behind them and slapped Phil on the back just a bit too hard to be friendly. "I'm gonna fuck you in the butt, boy!" he said in a gravelly voice.

"Grow up!" Marlie growled. "You used to be our friend! What's gotten into you?"

"It's not what's gotten into me," Tom smirked. "It's what's gonna get into Phil. Like my dick!" he looked at Phil. "I hope you picked a good girl name!"

"Grow up!" Marlie said.

"Like you did?" Tom retorted. "I heard that Andy tore you a new asshole!"

"It's 'Amanda' now," Marlie smirked. "It was by no means one sided."

Several months earlier:

Mark was feeling off. He felt that kinda buzzed, kinda antsy, and kinda horny feeling that some of his older schoolmates had told him about.

Mark noticed that Andy had a split lip from a fight he had gotten into earlier in the day. He also noticed that Andy looked really hot.

Their eyes locked.

Andy and Mark walked into a conveniently empty room next to the old shower rooms. For some reason, someone had left an old filthy mattress in it.

Both of them knew that they were likely approaching their rut, so they wore clothing that was easy to remove without tearing. Before anyone could totally lose control, they stripped. They had heard the stories. They knew better than to try to resist.

Mark was surprised when Andy took his penis in his mouth. He had expected Mark to immediately try to mount him. It didn't take the surprised boy long to blow his wad.

With a gentle touch, Andy encouraged the bemused Mark to get onto his hands and knees.

Unfortunately, the rut had riled Andy up beyond control. He mounted Mark way too quickly and thrust way too rapidly.

Mark struggled against the pain, causing Andy to slip out several times and have to roughly reinsert. This went on for what seemed like hours, but was probably only a few minutes. Finally, Andy ejaculated into Mark and collapsed.

When Mark went to the doctor, he wasn't surprised that he would soon be transitioning. He had resigned himself to that, since he was one of the smaller and nerdier types.

Andy, on the other hand, was surprised to find himself on the route to girlhood. He had received a dose of the enzyme through his split lip. It put him on the path to girlhood, but didn't clear the enzyme out of the reservoir of semen in his seminal vesicles.

current time:

Amanda, the former Andy, walked up and swatted Tom in the back of the head. "For the sake of everyone around you, I hope you get your rectum plugged real good! Being a girl will probably calm you down. Maybe you'll learn a few manners!"

"Just like you did?" smirked Tom.

Amanda rolled her eyes, then walked behind Phil and Marlie. She put her arms around both, put her head between theirs, and said, "You two really deserve to stay together. I really wish you had rutted each other."

"Yeah. Then maybe you would still be a boy!" Tom sneered. "But Phil doesn't have to worry. After I butt fuck him, I'll keep her and Marlie, too. They can stay together and take good care of me."

"As if!" Phil said. "You used to be cool, but you'll be lucky to find any girl to put up with your bullshit."

"I'm gonna fuck you in the butt, girlie!" Tom growled and walked away.

Amanda sighed. "Was I ever that bad? Maybe if I had been better, I would have been worthy..."

Marlie leaned her head on the arm that Amanda had over her shoulder. "You know that I still love you. And you know that you can join Phil and me. We used to have so many good times when we were kids. We can still be the Three Musketeers, you know.

Amanda smiled. "I might take you up on that. Just give me time to to get my head on straight, 'kay?"

"Remember what I said!" Tom's dad said as he took a swig from his fifth beer. "You make sure you stay my boy or I'm sending you back to your mother! I ain't raisin' no sissy!"

"Yes sir." Tom said.

Tom dragged his backpack up to his room and threw it on his bed with a sigh.

When did life get so sucky? Why did his mom have to leave? She is so weak! The last thing he wants to do is turn into a weakling like his mom.

But it happened to Andy and Mark. And Amanda and Marlie aren't weak like his mom. Maybe if he...

He rejected that thought immediately. The memories of his dad hovering over his mom as he beat her intruded. He never wanted that to happen to him.

"You used to be so cool! You used to be our friend!" he remembered them saying. Was he turning into his dad? Was he going to be like him?

For better than a century, the manly men boned the weak, and the weak became women. It used to be that you were born a man or a woman, but now, the strong turn the weak into women.

And he is going to be strong. He had joined the wrestling team to learn how to be strong and how to dominate his opponent. It didn't occur to him to use the term 'lover.'

And he is going to have two women. Or maybe three. And if they are good, he won't have to hit them. He doesn't want to hit them. He doesn't want to be his dad. Not really.

He wants to be strong, but he doesn't want to hurt other people. Not really.

Tom was feeling odd. He was feeling horny. He looked forward to finding Phil. Then, he would be a man forever and have three girls. Because he could tell. He was sure that Amanda wanted to stay with Marlie and whatever Phil was going to name herself.

Phil hastened to shower himself and get back to his gym locker. He could tell that he was about to rut, and didn't want to be taken by surprise.

If only he and Mark had rutted at the same time. He could have handled being a girl -- as long as he could stay with his lifetime best friend forever.

But now that Marlie and Amanda had changed, he had to stay a man for them.

He went way back to the end of the row near the wall and opened his locker. He hid behind it as he put on his chain g-string.

He had gotten some chain from a couple of dog leashes, and fashioned it into a thong. Butt floss. But instead of having a pad over his junk, he had a chain that went around both sides and met under his scrotum. A single strand went over his anus. It made pooping messy, but he made sure to do that at home every morning.

He held the waist chain together with a lock, snapped it shut, and left the key in his gym locker. His other keys were in his regular locker and at home in various places.

Tom had felt the rut coming on, so he skipped his morning masturbation session. He wanted to be ready to pin, insert, and inject. No fooling around. No wasting time.

Tom knew that his victim was around. It was like he could smell him. Soon to be her. He looked down the hall and, sure enough, Phil was walking his way.

Without words, they headed to the same room that Mark and Andy, now Marlie and Amanda, had used.

Phil barely had time to remove his clothes before Tom attacked him and pinned him to the ground. Tom expertly flipped Phil over, but Phil dropped to his stomach rather than staying on his hands and knees.

Not to be deterred, Tom tried to penetrate Phil. The fact that Phil's cheeks were together kept Tom from noticing that there was a length of chain between his cheeks. With the pressure from Phil's cheeks and the lack of lubrication, Tom didn't even realize that he hadn't penetrated. He blew his wad, but none of it even touched Phil's pucker.

Phil jumped up and flipped the spent boy over. He latched on to Tom's penis and sucked for all he was worth. Tom, not realizing that he hadn't succeeded in girlifying Phil, just smiled and accepted the blowjob as his due.

Phil spit the spooge out and used the bottle of water he kept in his backpack to rinse. No sense taking chances.

Phil sat between Tom's splayed legs, lifted Tom's butt into his lap, and latched on to his penis again. And again. And again and again, until Tom was exhausted and could no longer get it up.

While he was doing that, he had grabbed the small bottle of lube he had kept next to his water bottle and smeared it on his member.

Tom, his eyes rolled back in his head and his tongue lolling, barely noticed when he was penetrated by Phil's lubed penis.

Before he figured out what was happening, it was too late.

Phil grabbed his clothes and backpack, ran naked into the old locker room, and showered. He felt dirty, but exhilarated. He would remain a man for his two childhood friends. They would remain the Three Musketeers.

Tom woke up, remembering what had happened. He had managed to get Phil, and Phil had rewarded him with what seemed like dozens of blowjobs.

But then...

In a panic, Tom reached behind himself and felt the slimy semen right at his anus. He pulled his hand back around and looked at his finger. Then, he broke down and started crying.

Phil found him that way when, fully cleaned, dried, and clothed, he reentered the room to check on Tom.

"You... You bastard!"

Tom cried some more.

"I'm just a weakling! I can't believe you beat me! Dad is going to kick my ass!"

Phil suddenly understood what was happening. He had met Tom's dad, but hadn't been at their home since Tom's mom had left.

"It's OK. I'll take care of you. We'll all make sure that your dad doesn't get to you."

Phil took Tom to the locker room and led him to the shower. While Tom was showering, Phil collected Tom's clothes and backpack. He straightened everything out and left the clothes ready for Tom to put back on.

Even though Tom had been a real jerk for the past several months, Phil couldn't feel any ill will toward him. In fact, he felt protective. Loving, even. Was that part of the rut, left over feelings from their childhood friendship, or some combination?

Phil led the exhausted and sobbing Tom to the school office. He explained Tom's home situation to the principal when Tom was seeing the counselor.

Very soon after the principal called, Tom's mom was in the office. She wrapped Tom in a tight hug as soon as he walked out of the counselor's office.

Phil had a hard time eating with a girl on both sides. He didn't complain.

"So, all of our preparation paid off." Marlie commented.

"Yep, though it was a close thing. If it hadn't been for that chain jock strap, I would be heading for girlhood right now. And Tom would have ended up being a bastard like his dad."

Amanda smiled. "We would have liked you as a girl, but it's good to have at least one rod to bounce on," she said as she tweaked his crotch.

One of the lunchroom monitors gave them a glare, but hadn't seen enough to really be able to say anything.

Marlie smiled. "We didn't know what Tom was going through. I wish he had told us instead of acting like an asshole."

"Well, he's all better now," Phil said. "And it looks like he's going to be a good girl. errr... I mean, it looks like she will be good at being a girl. You know what I mean."

"Yeah," Amanda said. "Maybe he would have been an arrogant son of a bitch like his dad, or maybe he would have grown up. But she's definitely turning into a sweetie as a girl."

"And she's hardly started," Phil said.

"Trina is such a pretty name." Amanda said. And she's going to be really pretty once she finishes.

"I still can't believe he got us flowers," Marnie said.

"Except for me," Phil said with a theatrical frown.

"Would you rather have flowers, or the blowjob that she gave you?" Amanda smirked.

Phil gave a leer. "What do you think?"

Epilogue:

The queen sized mattress was spacious for Phil and Marnie, but it was cramped when they added Amanda and Trina. Not that any of them complained.

"You just like having three hot girlfriends," Trina smirked.

"I don't hear any of you complaining," he replied. And I definitely don't mind the show that you three give me when I'm recovering.