There is a reason there's a fandom joke of "Mamoru gets kidnaps by aliens. Who are in love with him." I could come up with nine canonical/heavily hinted at pairings for him off the top of my head.
Title: Little Black Dress
Author: purplekitte
Medium: Fanfic
Theme: Myths and Fairytales--The face that launched a thousand ships
Genre: Romance
Version: Anime, PGSM, musicals
Rating: R (warnings: mild sexual content, incest)
Kunzite had him pressed against the wall bruisingly, as usual. He was staring intently at him and Mamoru felt a suicidal need to ask if he had something caught in his teeth. Damn that spinach Mio had been force feeding him, he said silently, nonsensically following that train of thought.
“Don’t you dare smirk at me!”
Could he be so lucky that Kunzite would simply stab him already and get it over with? That would be a relief, honestly. Usagi would be upset but at least he wouldn’t be the universal hostage tying everyone down.
“What are you going to do about it?” Fear would involve a level of concern for his own life that he had never felt. “Are you all bark?”
It turned out Kunzite had bite as well, and he tasted his own blood as Kunzite kissed him, sharp against the ache and dizziness from his head being pushed against the wall too hard. Kunzite didn’t let up, putting as much passion into violence and sexuality as he did into violence and yelling or violence and... well, just about anything he did. There was already no space between them. He wondered if he’d fuck him right there against the wall.
Kunzite pulled away, breathing heavily. “I really hate you.”
Mamoru wondered if there was really a different between love and hate when someone wanted him that much.
***
“Kunzite had no success getting the Silver Crystal again. You should just remove him. He’s been no good for anything since you removed his lover.”
“You only say that because of his grudge against you.”
“That would involve feeling threatened, my sweet. We have more important thing to concentrate on.”
“But they don’t. A distraction until Queen Metallia’s revival is a fitting use in and of itself.”
“I suppose you always know best.” Endymion curled a strand of Beryl’s red hair around his finger from his resting place with his head pillowed on her chest.
“Just let me handle everything. Then they’ll be no more distractions to get between you and me.”
***
Ann wasn’t sure what was making Ail so jealous. They kept arguing whenever the topic arose, so she’d never been able to tell him about this fantasy she had.
It involved Ail, Mamoru, this Gundam doujinshi she had found in Tokyo, chocolate sauce, and a lot of life energy. She could already imagine Ail and Mamoru curled around each other and around her. She was rather proud of her planning ahead, really.
Didn’t he understand? She didn’t want Mamoru just for herself. He was her brother-beloved. They were made to share everything.
***
Longing, but pointlessly. She despised this weakness of the heart in herself.
“It’s good to see you, Setsuna. I trust you are well?”
He wasn’t smiling at her because he had special feelings for her. He was just being how he always was. “I have no complaints, my king.”
His eyes grew even kinder. “I do worry sometimes that you are reticent because we’re about to undergo some terrible disaster you cannot tell us.”
“I do not know how else to be.” She drank in his smiles like it was an oasis in the desert. What would his touch be like? How would it feel to be held by him? Maybe she could be different then, maybe she would be able to let go of herself. But it would be much too much and too little at the same time.
“I hope you will come to feel at ease in Crystal Tokyo. You are always welcome here.”
“I know, King Endymion. I know.”
***
“Mamoru is going to be living with us from now on,” her father explained to her.
The little boy didn’t move his gaze from the ground. He hardly seemed aware of her or her father or the maids moving his few bags into his new room.
“Mamoru? I’m Hina.”
“...Hina?”
“Yes!”
“He can be your new brother.”
“Brother? He’s going to be my boyfriend.”
“Boyfriend? Well, I could use an adopted son-in-law.”
***
Out in the cold dark, Fiore dreamed of roses, sleeping curled around the delicate flower Mamoru had given him.
It had long since frozen into crystalline ice in the shape of something that was once alive. He missed the soft kiss of the petals on his skin, but it was too brittle now.
The Xenian Flower shaped itself more like the rose at his heart’s request for the most beautiful flower in the universe. His hand slipped and he cut himself on a thorn.
***
“I’m a woman now. Do you like what you see?” Black Lady asked the mirror, rehearing for when Wiseman fulfilled his end of their deal and brought her Mamoru. He wouldn’t be wasted on a stupid, fat girl like Usagi.
The strap of her dress fell from one shoulder, bringing the neckline plunging even more precipitously. Black Lady measured the effect of the maneuver in the mirror, approving as her nipples hardened visibly under the thin fabric. She shifted her posture to show off the length and smoothness of her legs through the high-cut slits in her long skirt and pouted her full lips.
Too impatient to wait on plans and plots underway, her hand snaked between her legs as she imagined she already had Mamoru there with her.
***
Just because she’d seen him first didn’t mean Usagi got first dibs on him. Besides, everyone knew she liked Motoki at the video game parlor and she and Mamoru couldn’t stand each other. Usagi had no right to act jealous about Rei and Mamoru.
“I have a date with a mature older man,” she told Yuuichirou triumphantly.
“Are you sure about that?” he asked uncertainly.
“Yes, I’m entirely sure. Chiba Mamoru is the best there is and he’s just the guy for me.”
***
“I do!” Usagi shouted teary-eyed.
Habit prompted Mamoru to say something about her eye makeup but he couldn’t think of a thing. She was too beautiful, too radiant in the light of her white wedding dress and all her clear diamonds, but most particularly that of the ring he slipped on her finger. “I do too.”