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'.)
Usually a self deprecating term used to identify the person themself.
2) I'm a genderfuck, pansexual transfag.