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, hetrosexual, or bisexual.
Now, on to the definition of the slang word "Transdyke." It is a derogatory word used to describe a male to female transexual that has a sexual preference for women.
Technically, such a person would be lesbian, or homosexual.
This is similar to another derogatory term, trannyfag.