1. Right from the beginning, he established his role as the Alpha Male. When they got to the crime scene in the first episode, Inspector Lestrade asks, "Who is this?" And Sherlock repeats "He's with me". If Greg were also into men that would explain the subtle inflection that Sherlock isn't losing his new flatmate to this dense detective; thus, being subject to John recounting their dates all hours of the day as a result. Also, at the crime scene when Sherlock says, "Shut up," and Greg is all, "I wasn't saying anything," Sherlock was asserting his dominance over him in front of John.
2. He exercises control in all things, Miss Ste- (Oops, wrong fandom). Still, Sherlock uses knowledge as both his shield and his sword. When he loses control of a situation, it triggers a state of instability that requires a quick dose of insight to tip the scales back in his favor. Only when he is in complete control and assured victory does Sherlock maintain his mental footing. There's no telling what he might do if he loses the upper hand; that's when he is most dangerous.
3. For those of us who are into the phrase "Eyes of a killer"... He has those. Cold and calculating, picking up on every minute detail. Sherlock's eyes are the kind of steel that can pierce right through your heart, tear you into tiny pieces, and remain dry and impassive the entire time.
He even instructs John to inflict physical pain on him for a simple disguise. As far as a "self portrait" goes, what might we deduce about his self worth or value of life? The man is not afraid to die, faces his own death head-on more than once, and puts a bullet in the head of the man who threatens to dictate control of his existence.
4. Because you can't tell me that the path his eyes take is totally casual and not him checking out the goods. Sherlock is actually gay.
During their first adventure, everyone who has known Sherlock for years (years!), makes a comment that implies John is Sherlock's new man; his landlady Mrs. Hudson, the police officers who resent and ridicule him, a restaurant owner he paid a kindness, and his very own brother. Sherlock does nothing to dissuade this idea although he does make a point to establish that women are not his area.
When he was fake-dating Janine, he never slept with her and didn't show the slightest bit of compassion when revealing his plan was to break up with her from the start. He exploited Irene's feelings to win a power struggle. Molly's subtle flirtations were dismissed without acknowledgment. Even Jim Moriarty's obvious, pretend(?) interest was swept aside and forgotten until John became involved in the pool scene. It was a phantom sadist that intrigued Sherlock with a challenge; the only time he took an interest in someone besides John.
Yet, he never let anyone get close enough to become a liability until he fell for the man who wasn't even trying to woo his romantic attention.
If anything, Sherlock might just be "Johnsexual".
5. Do you realize that he ended this phone call, got that man (who has been professionally trained to defend himself and valuable political leaders) out of that chair, and shoved him through a window without the least bit of deception? The American was fully aware of Sherlock's intentions and still got thrown from the second story... how many times? I lost count.
6. His voice is like sex all on its own. Low and hypnotic; the kind of tone that keeps you grappling with the frays of consciousness while making sure you're so wound up in its dark promise that sleep is impossible.
7. From quippy one-liners to season long mocking, his sassy side is a force to be reckoned with and not a single person escapes its grasp. Sherlock takes sarcasm to a whole new level and his lofty views often shield the fact that he thinks the biggest joke is his own humanity.
8. They have that whole Dom/Sub thing going on. Sherlock bosses John around like an expert (and John obeys!). Even when he's in the flat robed with nothing but a sheet, John carries him around on the laptop, leaning down to the grass and talking to him through the microphone despite the gawking onlookers. John points out that it is "humiliating", and yet, does it anyway. John is an army man; and a doctor at that! He is used to taking orders only from his superiors (as a doctor he holds reign over most other soldiers) and here he is following Sherlock's whims.
In the first episode, he calls John back urgently because he knew where he was and who he was with; he tells John to give a text, to see how well he takes orders. John is resistant and a bit bristled by the request, but he does obey. His pride will make it that much more exciting when he is finally taken by Sherlock.
Sherlock insults John: "Because you're an idiot." But then tells him not to 'look like that' and offers a minor comfort; 'practically everyone is'. But, Sherlock doesn't apologize to anyone, right? So why make an exception for someone he just met?
"She said you get off on this."
"And I said dangerous, yet here you are."
He knows John gets off on the risk, the danger, and being put in a threatening situation. Just perfect for his sub, perhaps? ;) That would certainly provide a plausible reason for him to coddle the ego of the man he has designs on.
9. In a state of unfocused drunkenness, on the eve of John's wedding, his brilliant mind makes this connection. Most people would equate John to a masculine identity with the limits of alcoholic incoherency, or make a joke of it by putting someone like "Mother Theresa" or the name of the local nurse. But, to Sherlock, John will always be his sexy woman. *blushes*
10. Because he looks at John like this; with all the hunger of a pure predator.
BONUS: Because he also looks at John like this:
With so much affection the words "cynical" and "damaged" no longer apply.
