yeah, that M rating... this chapter is exactly why i set it that way.

for those of you who don't know, this story is also on Archive of Our Own. I'm posting it simultaneously in both places.

anyway please review! (tell me what you like, don't like, needs improving... help me become a better writer)

~inudigifan201

*this story's depiction of sex might not be accurate. it's hard writing a subject i know nothing about. but, hopefully it's not too bad.*

Ch, 4. The Mark

Inko had to leave the trip early to go back to work.

She gave Mitsuki a goodbye hug, Masaru a wave, ruffled Katsuki's hair and Izuku a mama bear hug when her taxi arrived.

"Behave yourself." She whispered in his ear before letting go.

He became confused by his mother's words. She knew he didn't have any behavioral issues. Wait… wait! Did she know? Did she know what he and Kacchan had done?

If she did it would make sense, she seemed to know everything.

He gulped. "Love you too mom." He waved her off nervously. Was he in trouble?

VVVVVV

Izuku and Katsuki were passed out in each other's arms. Both only wearing pull-ups. They were almost three years old and inseparable.

Inko smiled as she pulled a blanket over them to tuck them in for the night.

She turned out the light, but kept on a nightlight so neither of them would wake up and scream for fear of the dark. She left the door cracked as she walked into the living room of the apartment, grabbing her cellphone on the coffee table.

After finding the contact she was looking for, she put the device to her ear and waited for her friend on the other end to pick up.

"Hey Ko!" Mitsuki answered with a smile in her voice. "How's my brat behaving?"

Inko smiled. "Hey Suki. Katsuki's been an angel. The boys are sleeping now. They tuckered themselves out playing heroes."

"Great to hear. But I have a feeling you called to tell me something else." Mitsuki knew her well.

"Yeah." Inko nodded, then looked down at her sopping wet shirt. "Are you aware Katsuki's an omega?"

"Had a feeling, why?" Mitsuki informed her.

"Izuku is apparently an alpha like his father." Inko sighed.

"Oh shit." Mitsuki let out. "We're going to be future in-laws aren't we?"

Inko sighed again. "Well, Izuku did mark him. I'm sorry I was fixing dinner at the time and didn't stop him. They were playing and I don't think he realized what he was doing. And when I gave them a bath… I noticed when Katsuki was under the water..."

She cut her off. "It's ok Ko." Mitsuki was quiet a moment. "What does that mean for when they get older and he goes in heat for the first time?"

Inko pouted. "Omegas are rare, but I have seen a few at the hospital in agonizing pain because they couldn't get any relief… physically… sexually. They try to masturbate, but that does nothing because it's not the problem. Meds don't help either, they only prolong the pain. Almost all of them die from it." She bit her lip. "Their bodies are rejecting their own organs. Their white blood cells see the uterus as a threat and attack it in full force. If an omega has sex, then their body ignores the uterus. Nobody seems to understand why this is and research is almost nonexistent."

"So Katsuki has to get laid during his first heat or he could die?" Mitsuki sounded panicked.

"Unfortunately. Either during or before." A tear ran down Inko's face as she remembered those poor boys flat lining. She hated telling the families what happened to their sons. At least she could hopefully prevent Katsuki from dying prematurely.

"So we'll have to create a situation where Katsuki can get laid." Mitsuki understood. "Izuku marked him, so it would probably be for the best if it was him. It will be really painful if it was anybody else, right?"

"Yeah." Inko wiped her tears. "Besides, only Izuku will be able to smell him because of the mark. You and Masaru won't be able to tell when he goes into heat, but there are warning signs. And because of the mark Katsuki will crave Izuku's attention more than anything. He'll be the only thing on his mind. And Katsuki will be the only thing on Izuku's mind. Keeping them apart would be a very bad idea. If kept apart, Katsuki will subconsciously go looking for Izuku, not knowing what he's looking for and why, but if he doesn't find him in time... I'd rather not think about it."

There was a very good reason omegas were rare, a lot of them didn't make it to adulthood to reproduce. Being an omega was really a curse.

Although, for some odd reason, this information wasn't public knowledge. She reasoned a guess why, some unscrupulous people could take advantage of those poor boys, but she knew she'd see a lot less boys die if the public knew. But, that couldn't be the only reason.

The least she could do is save Katsuki… even if she got in trouble at work for it.

"Then we'll got our boys alone together when the time comes." Mitsuki's voice was confident, but shaky. Inko could hear her fear. And she hated it.

VVVVVV

The sun started to set as the boys reached the top of the small mountain.

They gazed out over the forest to the city in the distance while holding hands.

They had been holding hands since they left Kacchan's parent's sight back at the campsite.

The view was amazing, but neither of them were paying any attention.

Izuku smiled at Kacchan, but a pit was forming in his stomach.

His mother's words replaying in his head. Did they wake her up the other night? Could she tell when she checked on them? He hated hiding things from her, but he didn't want her to freak out. He'd have to ask her what she meant when he'd get back home.

Kacchan pulled him in for a kiss and he did as his omega wanted.

Huh. Kacchan was his omega. He still couldn't believe it. It almost felt like a dream and he didn't want to wake up. He couldn't remember when he fell for him, but he had fallen hard and had been that way since at least they were little kids. Although, when he started bullying him, he often wondered why he still loved him. But, it was nice to hear Kacchan tell him he loved him back, even if he didn't mean it. And he was pretty sure he didn't mean it. He was confident he just wanted him for sex… not that he was complaining. He was really enjoying the sex.

His tongue entered his mouth and made him moan as he wrapped his arms around his waist, pulling him closer. Kacchan wrapped his own arms around Izuku's neck and let him deepen the kiss.

It wasn't long until they were completely making out and their lower halves were betraying what they both really wanted.

Izuku broke away long enough to pull his sleeping bag out of his backpack and lay it out.

He then turned his attention back to Kacchan and had him pinned down to the bag. Their shoes and clothes all came off as they continued their tongue battle.

He slid in so easily, still surprised any of this was real. Surprised Kacchan was letting him have his way with him. Surprised he let him top and dominate him. Surprised he wasn't dead by now.

But he knew… he knew this bliss wouldn't last. So he'd have to commit every moment to memory and savor every last second. He had to memorize every moan and gasp. How he tasted and how he smelled. He didn't want to forget any of it when he went back to bullying him as soon as they got back home.

He knew what he said the other day, but Kacchan was horny and probably didn't mean any of it. He most likely only said it to get laid. Not that Izuku minded, but he knew it was going to sting when they got back home and everything would go back to normal. But, at least he would have this time to look back on.

"Izuku!" Kacchan loudly moaned causing Izuku to cum before he wanted to.

He wasn't done.

But the panting blond under him seemed to have had his fill… for the moment.

Izuku sighed as he dismounted him, laid down, zipped the sleeping bag up, and gazed up at the stars as Kacchan cuddled up to him. Yep, he was going to miss this.

"Izuku?" Kacchan rubbed his thumb on Izuku's soft chest. It was still a little strange hearing him call him by his real name, but he loved it.

"Hm?" He ran his fingers through his hair. He knew how much he loved it when he played with his hair.

"Everybody thinks I'm an alpha back home and you could easily pass for an omega…" He bit his lip.

"Yeah?" Izuku became curious as to where this was going.

Kacchan continued. "When we go home, we can still fuck right? But tell everyone, if they ask, that it's me fucking you and not you fucking me."

So Kacchan didn't want this to end either?

"Nobody has to know the truth." Kacchan finished.

Izuku thought about it a moment. He did want to be with Kacchan. He really didn't care what people thought of him, actually if everyone thought he was an omega, then maybe they'd be nicer to him since omegas were so rare. And the other bullies would leave him alone if they thought he was Kacchan's omega. So, win, win, win on all accounts.

"Sounds like a plan, just one problem." Izuku licked the spot where he marked him, unknowingly twice.

Kacchan moaned which caused Izuku to smile wide. He really did love that sound. He wanted to make him make that wonderful sound as much as possible.

"We could lie about that too." Kacchan sounded out of breath. "Say omegas mark their alphas. Nobody will know. We could make up anything."

He had a point. Nobody would question them and take whatever they said as fact.

"On one condition." It was high time Izuku got something he wanted.

"Anything." Kacchan seemed desperate.

"From this moment on, we're boyfriends."

"Deal." He got up, hovered over him, and lowered himself for a kiss.

Izuku laughed as Kacchan bushed. "Oh, you wanna top now?"

Kacchan's face turned a beet red.

Izuku smiled as his eyes narrowed and glowed. "I don't mind."

Kacchan flipped them and the sleeping bag over so he'd be on the bottom.

Izuku rolled his eyes and kissed him on the cheek. He then moved lower to his chest. He began to kiss, lick and nibble… leaving fresh hickeys.

All the while Kacchan made all the little noises he was really growing to love.

VVVVVV

Katsuki was in pure ecstasy.

He still couldn't believe the nerd wanted a committed relationship with him. He was perfectly fine with just being his sex toy. But, he asked him to be his boyfriend.

Izuku moved up to lick him behind his ear and Katsuki thought he was going to cum right then and there. But, the nerd had another plan. A much better plan.

He was inside him again. Each time felt like the first. And he couldn't get enough. He wanted more the more he gave him.

"Izuku!" He was now very familiar with orgasms, and he couldn't get enough of those either.