That was one thing I had to think about as soon to the outside world I will be in a same sex relationship. I have never thought of myself as a lesbian and not sure I ever will. But I also know that I love my partner no matter what gender they are. And so don't care what people think of me as, as long as they know that I'm in a happy, loving relationship.
I feel in love with her and not what is between her legs.
I'm sure it's similar to your partner. They love you for you and as you transition you will become an even better, happier person.
It will be hard and each step of the way you will take it in stride but just make sure to talk about it openly and don't push him into anything sooner then he is comfortable with.