Okay so real quick I'm going to answer a statement one reviewer made. I'm sure it was bugging my other readers too. Bugged me too till I figured it out.
Yes, Mycroft is somewhat passive in their relationship. I know he's pretty proud and self-reliant and all that fun stuff in the show, and here it seems he's, well, more of a scared kid. But who isn't afraid when going into their first relationship? He doesn't know anything about this, and not knowing scares him. It scares me. Plus, in the government position Mycroft has, he would have to be the epitome of confidence and certainty, presenting his solutions as the only correct one. But nobody is always confident and certain of themselves, regardless of what others think. I'm trying to portray Mycroft as an actual human being, afraid of the unknown, uncertain, that sort of thing. In all honesty, this story wrote itself. I had no part except typing it out.
Another thought that just occurred to me. I think maybe who I was thinking of when I was writing Mycroft was myself. First off, I've never been in a romantic relationship. Secondly, I appear very confident and composed on the outside, so much so that everyone assumes I know it all. Really, though, I freak out a lot. At least once daily. There are only a few people I show that too. Maybe Mycroft's the same way. It's hard to trust someone that much, especially if you've had your trust broken by somebody you really trusted.
And there are times when you really just need to let control go.
So, there you go. A three paragraph answer as to why Mycroft is fairly passive in the relationship.
EDIT: Just went over my stories again, read this, and started giggling. Goodness I really waxed philosophical or something!