See, there are millions of types of relationships, billions of them going on right now. Start-ups, break-ups, make-ups, failing ones, honeymooner ones, boring ones, oversexed ones, creepy ones, abusive ones, inclusive ones, awkward ones, and then, the lucky few, the perfect ones.
The ones where the participants are so tailored to each other's wants and needs that they just can't not be together.
This is the story of how one of these couples work.

Sherlock Holmes is a brilliant, maniacal, and great man. He's handsome, smart, mildly famous, connected, and definitely makes good money. He's the man you take home to Mummy.
The one hang-up? He doesn't want sex.
Love, yes.
Sex, no.
So, most relationships with him end up as break-ups, fails, or awkward ones.
Except for this one.
Doctor John Watson is a good man. Queen and Country, loyal, protective, clever, sweet, devoted, and all-in-all, the guy you want for a good hug and a cup of tea. He is not a homosexual, meaning he is not sexually attracted to men.
Not. Sexually. Attracted.
He wants to sleep with women, but he never loves them.
He's romantically attracted to men; well, one specific man. Sherlock Bloody Holmes.

"After compiling all the sensory data, the only reasonable deduction is that I am in love with you," Sherlock said, peering into a microscope. "And, yes, I'm speaking to you, not the flake of skin with cyanide poisoning."
John sets his teacup down harshly, eyes wide. "What?"
"I am romantically attracted to you. I am in love with you. Is that not how people say it? 'I love you'?"
"No, Sherlock, that... that is how people say it. Why-?"
"Elevated heartbeat, dilated pupils, heightened blood flow to the facial capillaries, etcetera. The signals are quite clear, John. I had always thought it would be rather dull, love, but it really isn't."
"Sherlock, I know I have to constantly announce it, but I am not gay."
"I don't want to have sex with you."
"That's not the bit I'm objecting to-well, yes, a little bit-but I don't like men!"
"Sexually. You've never been in a purely romantic relationship with a man. Or anyone. All I ask is a week. One week for me to prove that we are actually a rational choice for a non-sexual relationship."
One week later, John is the big spoon and Sherlock is sleeping in his shirt.

So, do you now understand?
Sherlock doesn't want sex; John doesn't want it with him (specifically, with his body). Both want a romantic relationship.
Of all the relationships in the world, they are the lucky few.
They work.

This is my take on Johnlock - as much as I love a good ole Johnlock romp in the bedsheets, Sherlock's never come across as a sexual being, and John sleeps with his girlfriends, but never tries hard to make the relationships work, always dropping things for Sherlock's needs.

So, I decided to delve into the different types of attraction - John likes sleeping with women, but never loves them, and it is commonly observed that he loves Sherlock; Sherlock isn't sexually interested in anyone (though S1 e1, he ogles John's arse the first time he steps into 221 Baker St.), but John is the person he cares for - Moriarty uses John to get to him, and he's the first person that comes to mind when "friends" are in danger, and John's his only friend.