Trannyfag is only a derogatory word if someone doesn't want it applied to them; the same as the word fag. That is, one probably shouldn't use it to label other people without their permission, but lots of people use it to label themselves, in which case it is not deragatory, but the reclaiming of a word that was once used against them (much like fag in general - and in this case that's the reclaimed word since trannyfag was never a popular insult for androphilic FTMs, as far as I know, - nigger, etc.)

Also, sexual orientation identity is not necessarily dependant on the person's gender identity; in fact, it only need be if one identifies with the classical sense of homosexual and heterosexual. Bisexual, asexual, pan-sexual, polysexual, intellectsexual, queersexual, etc. (as much as your mind can come up with) people can be any gender. This is part of the beauty of the label "trannyfag": while it does indicate that you are a tranny, and you like dudes, it doesn't peg you in a gendered role the way that "fag" or "dyke" do in and of themselves. Even the identity "heterosexual" implies a certain level of gender normativity, though it doesn't specify which gender normative identity its referent possesses.