I don't know exactly what will happen to my relationship (10 yrs and counting), given that my partner is male and not in the slightest gay or bi, but I can take a fair guess. It's going to become a mutually supportive friendship for a while at least. I don't miss sex, but he will. So I don't know how all that will work out. He's not very flexible in terms of what he wants to do in that dept. so I figure he knows what's coming. If he chooses to leave the relationship I'll be pretty deflated, but not surprised. Like other cis people, he has almost no idea what it's like to be trans and how flexible and accommodating you must be just to have some semblance of a normal life, and unlike me, places less emphasis on the person and their personality in a relationship and more on the sex and the looks. Not a whole lot compared to most people seem to, to be fair to him, but much more than I do.
I will always be his pal and support him because that's how I got into this relationship in the beginning, and I made promises to look out for him, and he says the same. We've already been through the grinder down the years and somehow nothing seems to make us want to go separate ways at all. Me transitioning will be the ultimate test of that, I figure. The longer you stay in a relationship, the harder it is to walk away from it I suppose, with all the investment and shared experience put into it.
All my relationships eventually seem to end up platonic. I think that was just a consequence of my pre-transitional predicament. I do not get into these situations for the sex so that aspect always breaks down, but the basis of the relationships is always a decent friendship so it never falls apart completely. And it's a strange thing to hear stories from my ex or someone I was in a platonic thing with nearly 2 decades ago that when it comes to their partners since, they never seem to last long, they always seem to have issues with jealous or vengeful exes. I've never experienced that as I never go into relationships for sex or without someone I know well enough to trust. Maybe it's a bad thing to be so attached though, I don't know. I feel kinda bad for my partner because he could be happy with some hot female right now and I woudn't be responsible for him feeling glum about the fact he isn't. But he is free to walk away any time if that's what he truly wants. I will be sad, but I am not his keeper. I think our relationship changed the moment I told him... now it is something else and it can't go back to what it was.
I guess I've become too tired to fret it any more. I have asked to talk about it dozens, maybe 100s of times with him and each time it's the same answer. Slowly I am starting to think of myself as (somewhat) single again, in order to prepare and protect myself, just in case.