I would say that you'll need to do some serious soul searching on this. Are you attracted to your partner for physical reasons...as in beauty, body,... or was it about "her", the person?

You see, your partner (as a woman or a gay man in a woman's body) is attracted to you. A change is your partner's body will not change that attraction. As for your partner's body...will some changes make you not attracted? For example, most FTM people will not get "bottom" surgery because it is extremely expensive and doesn't provide nearly as functional a result as a vaginoplasty. So maybe your partner gets the breasts removed to fit closer to the correct body image...is that any different from having them removed due to cancer?

My point is that if you love your partner, some appearance changes shouldn't change that love. Does it matter that in your partner's mind, "she" is a "he" and that "he" is making love to you as a gay man, not a straight woman? I know it isn't that simple, but physically you will likely still be able to have vaginal intercourse with "him", and "he" will pleasure you the same no matter how your partner identifies.

I know that there a plenty of couples that have stayed together after one has transitionned, because they were friends/partners before and they saw no reason to change their relationship after their body image was altered.

Good luck.