I've not ever dated a lesbian, but I know a guy whose longtime (now ex) girlfriend of seven years was a lesbian. Near the end of their relationship, he came out and started to medically and socially transition. He started with top surgery, and I remember her leaving him without warning for almost three weeks after because it broke her emotionally to see her butch girlfriend becoming a man physically. They talked, and went to counseling and tried to make it work for a long time. He tried to put off some parts of his transition for her, to make her feel more included and comfortable, but eventually started T because he felt it was a big and very necessary step in his transition. She said she would not freak out and helped him with his shots for a long time, but...As soon as he was passably male, she broke it off with him. She said that while she loved him as a person, she wasn't straight and couldn't be that way for him, just as he couldn't pretend to be a lesbian woman with her. They were both sad, but they also talk about how it was probably for the best that they don't hold down their partner in a situation like that.
Generally, that's what I've seen happen with many lesbian/trans male couples. Sometimes sexuality is not very fluid, depending on the individual. She liked him as a girl, but no matter how hard she tried to support him as a man during his transition, she couldn't remain attracted to him. She liked girls, he obviously wasn't one.
One way to address it with your girlfriend is to be open to her possibly being unhappy and letting her talk about it. If you are both very open about how you feel about things, there's honesty and more of a foundation built. Your girlfriend may have to deal with the possibility of people viewing you as male(and both of you, by extension, as a straight couple). Can she handle that? Is she open to change?