We heard quite a lot from Stephanie in this interview in 2020. (Interview is transcribed in 4 parts, link goes to part 3.)
Stephanie had no say at all. Debbie said he would leave if Stephanie wanted him to. Eventually she asked him to leave. He refused. Then Debbie gave Stephanie a tiny space to have her input.
https://www.mumsnet.com/talk/womensrights/4053058-Debbie-and-Stephanie-Hayton-interview-transcript?reply=100925534
Kristen [interviewer]: Did you think about leaving at any point, did you think about divorce or was it, 'let's keep it together for the kids and the family'?
Stephanie: We decided, I can't remember why, but we decided that maybe we should take time out to chat once a fortnight or so and we went out to a local arts centre on the Thursday evening and we would chat about how things were going. And Debbie's life at the time did revolve around Debbie. It was what Debbie wanted; Debbie was choosing. I had no say apart from at maximum, 'Just kind of slow down very slightly' and one of these Thursday evenings, I just said, 'Well look you keep telling me that you're going to transition, you're going to keep going. Even if you have to leave, even if you have to walk out, I think maybe we need to separate because I can't cope with it anymore,' and Debbie said, 'Okay if that's what you want, then that's what we'll do.'
But my daughter was about two months away from her first set of significant School exams, which are taken at age 16 in Britain. And so two weeks later, we went back on a Thursday evening to this local arts centre and Debbie said, 'I've been thinking and I'm not going to move out. I want to stay' and at that point, I really just felt, 'Well, I've got nothing here. All along, you've been saying that, if necessary, you will leave and eventually I've got to the point of saying okay, you need to leave. And you're now saying you refuse.' So again, actually that was a turning point for Debbie. I think Debbie realized that if she wanted to stay within the family, she needed to actually start thinking about the family a little bit more, rather than just what she wanted and how she was going to do things. And after that, I started having a little bit more of a say and Debbie would ask my opinion about things and I felt that my opinion was actually making a small amount of difference, maybe not huge, but a small amount of difference. And we got through my daughter's exams and, at that point, you know, it was then three months after I'd said, 'I think you need to leave'. And things were better, though a long way from perfect but much better than that very, very low point.