Hmm, well, I've yet to tell anyone but friends that I'm planning to transition soon, but I imagine this sort of 'man up' response is in the same vein as what gay FTM's experience. "If you want to be with men, why not be a girl?" seems remarkably similar to "if you're going to have 'girly' hobbies, why be a boy?"
I've also got some 'feminine' hobbies. I make costumes to wear at sci-fi/fantasy and anime conventions, which would include sewing (although I've honestly never enjoyed the sewing parts), I occasionally collect Pokemon plush and toys, and I enjoy musical and non-musical theatre. On the flip side I have more 'masculine' interests as well. I follow college basketball, I enjoy crafting with power tools, wood, and plastics such as building costume props and armor as well as DIY home repair, and I build and fix computers.
I do think a few of my more effete interests will garner me some grief at some point during or even post transition, but I'll just laugh it off because I personally don't plan on pretending that I had a typical 'boyhood' because clearly I didn't. There's always going to be some residual quirks that'll be noticed, and if there weren't then I'd be hiding aspects of myself, and that's why I'm transitioning; to stop hiding how I feel and what I am. I'm not looking for a history re-write and to pretend that past experiences didn't help make me who I am, and aside from a few physical things, I quite like that person, haha.
Besides, I think gender variance can be awesome when people are happy being who they are and that stereotypes like 'boys don't play with dolls' and such hurt everyone, not just those in the LGBT spectrum.