A transsexual (Usually a transman: a man whom happens to be transexual) whom is attracted to members of the same sex. (NOT the gender they were born with, but the gender they have become/are on their way to being).
This means to a transman: he will date biomen and transmen.
This means to a transwoman: she will date biowomen and transwomen.
It is generally a term used by trans people themselves, to describe one another in jest (exactly as many homosexual youth call each other 'fag' nowadays). Transfag is a combination of 'trans' (meaning transgender or transsexual. transgender being somebody non-gendernormative, and transexual being somebody whom changes their body permanently) and 'fag' which in this case refers to homosexuals.
Also, for further reference, it is generally EXTREMELY offensive to refer to a transsexual as the gender they were born with as opposed to what they are. Especially when the offending person knows what gender they are (i.e. 'Margeret, whom wants to be Lee' and then calling him a 'she' would most often be exceedingly offensive to 'Lee'.)
Sam and Nate identify as transfags because they're fags whom happen to be transexuals.