Life is very short, make and do what ever you want if it is possible for you.
Shemale/T-girl experience is a fantasy which you can not live everyday. It is possible talk with your girlfriend about this but her reaction could be good or bad. But all i can say is if you want to do it, Do It and don't think about if i do it, i ll be gay. thats bullshit !!!
I have a friend and he has a very good word.
"try everything once inlife. if you like it, try again"