I've also thought about this a lot, and come to the conclusion I just don't know. If my husband all of a sudden announced that he was going to transition and wanted my support, I doubt I could give it. There would be a major trust issue, and he would have demonstrated he was prepared to put himself before me. If I wasn't open to an asexual or lesbian future, I think I would find moving on to another hetero relationship attractive.
But if my husband was open to me before we got married, demonstrated a history of regarding me in his decisions to build trust, then included me in the problem solving around his gender issue, I would feel more certain of my future, and maybe, just maybe, willing to support him and find the best outcomes for us both. It would depend a lot on age, children, financial situation, family, careers, and unrealised dreams.
It is so hard to predict how I would react in so vastly different situations, but the basics of trust, love, and future needs would be the guides.
Allie