John & Sherlock vs. John & Mary

With Mary, John is forced to become a bottom. With Sherlock, he is allowed to be whatever he feels like being, because they’re partners.

This applies to every aspect of the comparison. When Mary and John “banter”, it’s one-sided. Mary “playfully” says what she wants to John and John receives it until he gets seriously angry. With Sherlock, there’s give and take, and in the heat of the moment, whichever of them is more qualified to take the lead in a situation takes it. If it’s a dead body, Sherlock is in charge. If it’s a live body, John is in charge. There is space in their relationship for that, for each of them to fully live in their own skin and be who they are. Mary limits John. She chips away at his confidence (or tries to) and belittles him. Compare this: 

Sherlock: Nice touch.
John: Haven’t pulled rank in ages.
Sherlock: Enjoy it?
John: Oh, yeah.

To: 

Mary: Seriously??
John: Why not? She’s not going to the police. Someone’s got to get him.
Mary: Why you?
John: I’m being neighbourly.
Mary: Since when?!
John: Since now. Since this exact minute.
Mary: Why are you being so …?
John: What?
Mary: I dunno. What’s the matter with you?
John: There is nothing the matter with me!

Call that “banter”, if you want, but that’s not what it sounds like. And that’s just one tiny section of an entire scene of this, of Mary questioning John’s abilities and motives, ridiculing his choice to take a tyre lever with him and correcting him in front of their neighbour. Why NOT Captain John Watson of the Fifth Northumberland Fusiliers? Who better to effect the rescue of the neighbour’s kid? 

Sherlock, on the other hand, gives John everything he needs to be who he really is. Mary wants John to fit into some little box that only she had a say in designing and she calls John out whenever he steps out of it (”I’m taking Mary home.” / *Mary clears her throat pointedly* / “Mary’s taking me home” - MP, yes, but this is clearly Sherlock’s observation of how their relationship works.) Sherlock, within twenty-four hours of having met John, restored his ability to walk, gave him the adrenaline he craved and cured the tremor in his left hand, allowing him to return to medical practise and take whatever part he wanted in Sherlock’s casework at the same time.

One of these relationships gives John freedom and affirmation of who he is, what he wants, and the means to be and do all of it, and the other gives him a cage and a role to play with someone else who is also playing a role. These relationships are very, very different.