Thanks for asking Alterheros your question. I understand that when you discover something that might appear to be unconventional and out of the ordinary about yourself you might start to wonder and worry about where this thing came from, what it means about you, and when it will go away.
Before I continue though, since some people do consider the term ‘she-male’ to be pejorative I will be using the term transsexual in answering your question (I understand that we are only talking about male to female transsexuals who still have male genitals).
It is common for people to fantasize about something that is considered to be taboo, risky, and different. In particular, straight males fantasizing about transsexuals is so common that the book ‘Guide to getting it on’ by Paul Joannides (2007) features a section about it. Just because you fantasize about something, does not mean that you want to do it yourself. Fantasies are often best kept that way, and many times when fantasies become real, the result is not at all what had been expected (think about someone who fantasizes about a three-way but feels uncomfortable and unpleasant when they actually decide to go through with one). Fantasies are often just that, nothing more.
In short, there is nothing wrong with you. If you enjoy it, if it works for you, if it isn’t hurting anyone then no, I don’t think you need to necessarily stop watching porn with transsexuals. What you might want to consider doing is talking to your girlfriend about this. It might make you uncomfortable and nervous, but talking about sex and your desires with your partner can be a really intimate experience. Especially if she has already been suspecting it, you might help her and be able to answer some questions she might have. Maybe she is confused, and even worried that you are hiding this from her. Open communication can be a rather important part of a healthy relationship. Also, being open with your girlfriend about this fantasy might open up some doors for you to explore it some more if you want.
I hope this helped. Good luck, Chris! Let us know if you have anymore questions.
Hillary, for Alterheros