Sherlock hates John's ex's.
"Sherlock, you realize you've never met him, right? As a matter of fact, you've never met any of my ex's."
"I've met Sarah."
"Yeah, but-"
"And, quite frankly, I don't like the relationship you have with her."
"What relationship? She's my boss!"
"Why are you shouting?"
"Because you're ridiculous."
"I just don't like him, John."
"You don't even know him! I dated him seventeen years ago!"
"He was your first boyfriend, wasn't he?"
"Sort of."
"Wh-what does sort of mean?"
"It means…yes, yeah. He was my first boyfriend."
"I don't even want to know what you're hiding."
"Sherlock, you realize that I have had sex with other people besides you? And I've had other relationships besides you? And that, even though all of that is true, I've only ever loved you?"
"So?"
"So you shouldn't be worried."
"I'm still worried."
Well, John's 'First-Boyfriend' came a week later for a visit. John hadn't seen him since before the war, well, since before John went into combat.
His name is Kent Smith. He was really pretty, if Sherlock was going to be honest with himself. He was just pretty. He was handsome, yes, but he was far too well groomed to be 'hot' and far too feminine to not be pretty.
Sherlock wondered if John liked pretty boys, and if he was a pretty boy. He was rather feminine, after all.
Kent was really nice, too, which just punched Sherlock in the throat even more. He was nice to John and he thanks John for every little fucking thing, and he complimented John, and he asked John questions about the war and his current life.
John was straight with Kent when he told him Sherlock is his boyfriend. Kent asked them questions about their relationship.
About marriage.
Maybe one day.
About children.
Definitely not.
About pets.
Just Gladstone.
About how it happened.
By accident.
About Sherlock past.
Adopted. Killed animals. Massive intellect. Almost engaged. Found John. Life in bliss.
Those were his words.
Later, they sent Kent on his way to his hotel, the nice one John picked out.
And Sherlock attacked John and dragged him to bed.
"She-Sher-Sherlock! What are you doing?"
Sherlock pulled John's pants off and was in the process of pulling his own off, "Claiming what's mine."
"You're insane." John's breath was shortening and getting heavier, and that was possible.
"Do you want me to stop, then?"
"No, no, no, no. Yours, yours, yours." John was nearly panting already.
Kent returned to the Holmes-Watson residence the next day. Sherlock was more unhappy today than the day before, and Kent was trying harder. And John was giving in.
The same thing happened that night than the night before.
The next day, Kent returned for his last visit. To cut the story short, Sherlock left the room and Kent made his move.
"No, Kent, I have Sherlock, I don't-"
"That's ok, he doesn't have to find out."
"No, but he will, and I don't want to. I'm-"
"Come on, John."
"No, no-"
Sherlock, as quiet as he can be, came into the kitchen. "He said no, Kent."
"Oh, Sherlock, I-"
"No, I know what you were doing. And I don't blame you, John is brilliant. He's smart, he's funny, he's caring, and he's really, really sexy. But he's not yours, anymore. So, if you'd please leave-"
"Look, John, I-"
"No, Kent. Leave." Sherlock sounded scary.
Kent went to the door. "John if you-"
Sherlock grabbed his coat collar and pushed him out the door, "Yes, thank you, Kent."
"Sherlock, I'm so sor-"
Sherlock kissed John. He kissed John and reminded John that John is his. His.
"I love you, John. No more ex's. Please."
"Deal."