You mean, are SOs who stay a rare breed? Quite possibly, but I just think that most SOs in that category have more protean sexuality than the strict gay/straight binary that a lot of now-absent SOs seem to have. Only anecdotal evidence to back me up (on Susan's and my own life), but it's quite a few anecdotes that SEEM to add up.
My ex was completely straight, no middle ground.
He claimed that if our relationship had been stronger when I transitioned--and it was clearly MY fault that it wasn't--then he might have been able to ride out my transition and stay with me.
I take plenty of responsibility for the increasing weakness of our relationship, but it was a two-way street; and I think his explanation of why he didn't stay with me is a load of crap. When I look back on the strongest years of our relationship, I can clearly see that he completely shut down whenever I "actively" tried to pass, whereas he was okay when people simply "mistook" me for a man. The former turned him into a stiff, uncommunicative little zombie, and the latter was all a big private joke that he could laugh off, presumably because he "knew" the "truth."
In my circle of in-person acquaintances, I can immediately come up with five relationships that have survived transition so far. In two, the SO is bi, and both relationships have survived complete transition (as far as either trans person has wished to go) and have been going strong years and years.
In another, the SO still calls herself a lesbian but seems completely committed to her trans man. He hasn't had top or bottom surgery, but I have little doubt that they will stay together. They've been together for at least five years, and they love each other like crazy.
In another, I don't know the orientation of the SO. Her trans partner is on hormones but hasn't had top or bottom surgery. I have to say that it's a very disturbing relationship. The SO is overbearing and even somewhat abusive. Some of us wonder if the trans person stays in the relationship out of fear of being alone and the non-trans partner stays because she likes to control and dominate. It's not a mutual b&d scenario. And, of course, there's no telling what will happen when the trans partner does have surgery.
A fifth relationship required no sexuality shift for the SO. She was straight, she saw her partner as a man, and they got together while he was still pre-transition. He finally had top surgery recently. I gather from conversations with the SO that negotiating her partner's presurgical chest required a great deal of work and negotiation, so I imagine they are both much happier now.
I guess that's kind of a mixed bag, isn't it? But I wouldn't be surprised to find out that more than fifty percent of the "staying" SOs are bi or pan.
Then there's the other side of the coin. Some trans people leave their SOs because of transition. I can honestly say that if my ex hadn't broken up with me when he did, it's quite possible that I would have broken up with him not long after--in perhaps a year or two. You see, I wanted to be with a gay man, and he made it quite clear that he wanted nothing to do with that kind of world.
Somewhere I saw a reference to a study about SOs, but I never followed up on it. I don't remember if it was FTM or MTF relationships, but I think the former. There could be several studies out there by now. It would be interesting to see whether bi/pansexuality plays a major role in most such relationships.