First, it should be noted that in the transgendered world, sexual preference is based upon two things: The gender identity of the person, and the gender/gender identity that person is attracted to. They may be homosexual, heterosexual, or bisexual.
Now, on to the definition of the slang word "Trannyfag." It is a derogatory word used to describe a female to male transexual that has a sexual preference for men.
If one wants to truly be enlightened, one may wish to consult the Transgender FAQ: http://www.geocities.com/westhollywood/6888/tgfaq.htm