The truth of the matter here-and I know that there are those who will deny even this-is that unless we have been through this situation we do not know how we would react. As a bisexual man, even I do not know for certain how I would react.
Some people are more into the actual person-emotions- than into just the sex. Add to this mix, a matter of how long a couple has been together, their age, and perhaps a gazillion other factors. Would the couple be more likely to remain united if an outside sex partner were allowed for the one who did not have the gender change?
In 1998 I met a man who was a man (no, it was not sexual). He was a charming cheerful and happy man. All appeared to be fine on the outside. Then he announced his intentions of becoming a woman. I was somewhat surprised, even stunned. It hit me out of leftfield. Over the next few years he did all the necessary things towards becoming a woman. He let his hair grow long, took hormone injections, wore women's clothing, went to thousands of counseling sessions. In 2003 he had phase two, when two massive breast implants were installed. That was when the friendship began to have problems. He (still had the penis) became obsessed with the damned breasts. That was everything. Well, that and the damned hair. Every place he went, out came the brush and the brushing of the hair began. It was a phase.
In 2005 he had the final cutting (say goodbye to the penis)and reconstruction (well, technically original construction) of a vagina. Gradually things began to improve and she became more of her old self again. Charming, cheerful, happy, giving, caring. My friend, now a woman, a beautiful and confident woman, was back. If I had abandoned all connection at the start there would have been nothing. It taught me a lesson as well. I found within myself the knowledge that, if I truly have a bond with someone, it cannot be completely broken by something such as a gender modification. If they are someone I really care about, then shouldn't I also care enough about them, love them enough to maintain some relationship with them? Shouldn't I be supportive of them? Is turning and racing out the door supportive? Most definitely it is not. I saw persons walk out on my friend when he announced his intentions. Is that how anybody wants to be remembered? Is that any different in essence from the haters of gays, blacks, Hispanics, Jews, or any other group of people?
Again, I must indicate that I do not know that I would stay in a physically intimate relationship with a man or woman if they felt the need to do this. I will however say, that I would not totally abandon them in the manner which so many- even in this thread- seem willing to do. If you can do that, then the relationship was doomed towards failure from the start.