*What makes you think her 'soul' would stay the same?
I've mentioned before those who have said Yes have made it all about the genitals, those who have said No have talked about the lies, the gas lighting, the change in personality, the change in sex dynamics, the re-writing of history... I could go on.*
I feel like this argument sometimes takes place between two extreme points of view, neither of which is necessarily accurate. My experience is that the "if you love someone then you shouldn't mind, because you love the soul, not the genitals" is not a very realistic portrayal of relationships full stop. What even is loveable about someone's soul? That isn't how reality works - in reality, big life changes put a huge pressure on relationships, whether that's your partner deciding to quit their career as a banker to become an artist or if they became disabled and needed you to be a carer or found religion or whatever. And gender transition is a massive change that, in my experience, would put a strain on any relationship. The "if you love them you'll just be happy for them" narrative isn't fair and does silence partners when they also nearly always need support.
Equally, I am not very sold on the assumption that transition makes a total lie out of your whole past relationship or involves a total change of personality etc. That wasn't what happened to me or any of the other couples where one half was transitioning. My now ex and I still talk about our relationship, I've never been told I'm not allowed to remember it. She didn't totally change personality - she is the same person under it all - she still has the same academic interests and hobbies, she still has most of the same friends, and the focus on hair and make up settled down hugely over time - it was just a big deal as she was having to learn so much that natal women tend to pick up slowly over time, normally in our teens, and then can discard or keep as we want.
I wish it was possible to discuss the "in the middle" a bit more. I ended up feeling like I couldn't express the issues I was having with her transition while still caring about her without being called names by one side or the other and that massively sucked.