1. the sexual orientation of a person (otherwise considered to be 'straight') who is gay because of their involvement with a transgendered individual of the same gender identity.
2. a man who is attracted to trans-masculine, masculine genderqueer or FTM individuals, and thus gay with reference to transguys. Similar to a tranny-fan but in reference to FTMs in particular.
I considered myself to be straight until my girlfriend identified as a transgendered male...now instead I am transgay.
Me: I can't get excited about those bio-guys over there but that transguy is so hawt! Think he's a trannyfag?
Bob: You're such a transgay.