Couples in relationships change and sometimes go their seperate ways for all sorts of reasons, no one can predict the future and the possiblity that a relationship may end is always there. Using your being TG and the chance that you may "turn gay" (unlikely, but who knows) is, it seems to me, and I don't mean this to be disparaging of your SO, simply reluctance to commit. Everyone takes a chance when they enter into a relationship that it may all come crashing down in the near term or years from now, if it were a valid reason, then no one would ever take the chance and there would be no relationships at all. This is just my two cents.
Like so many of the other girls here, my being TG and my expression of it has taken paths I would never have thought it would, oh say, ten years ago. For one thing I never thought I would ever go out as my female self and feel so good about who I am. FWIW, it has not destroyed my relationship with my wife and while we have had our rocky patches, in many ways it has strengthened our marriage. Good luck to you both, I wish you all the happiness in the world.
Hugs...Joni Mari