Some have touched on the thoughts I have here, so in those cases you can just add my support to those that have already mentioned them. And I will elaborate on some more.
We fall in love with the image we have of the other person; it encompasses all we know of them at that time, in addition to traits, behaviors and qualities (good or bad) that we 'assign' to them based on how we feel about them. And we fall in love with that image. One of the most important components of romantic relationships is sexual attraction; and yes, a relationship can survive once that attraction wanes, but usually not if that attraction turns to repulsion. And that is what can easily happen when your masculine image turns into a feminine image. Not all women are completely sexually repulsed by another female, but a good percent are. It's not something we can pick or choose, it just feels very wrong for many of us. And women can feel the same way. So when she first imagines or sees you as a female, her sexual attraction to you can be disturbed or in many cases completely destroyed. And for most women, romantic love is the very key that holds their relationship with you together. Once you disrupt that, it can be the beginning of the end. Sometimes the relationship changes and survives, sometimes not. Some women can compartmentalize your crossdressing from the rest of your life, others cannot.
In other cases, sometimes insecurity comes into play. A woman may become upset, for example, if her male partner's breasts become bigger than her own Or if when 'en femme', he looks prettier than she feels she looks. It simply threatens our image of who we are in the relationship. Yes, it would be nice if what our bodies looked like didn't matter at all, but unfortunately life doesn't work out like that.
I've pondered over the issue of not telling someone early in the relationship about my crossdressing, on how my ex wife complained about that 'lie' of omission being the most upsetting thing about it, that it was my dishonesty that upset her more than anything else. And, I don't think it was, I think she expressed that because it was more politically correct to say that than to complain about the crossdressing in and of itself. After all, there were plenty of other things that we never mention about ourselves, because we think they really aren't important; she, for example, had several issues as well that she never mentioned, but as she felt they weren't important, she didn't feel it necessary to tell me. I felt the same about the crossdressing. And so, I don't think the 'dishonesty' argument really holds water. After all, we are all liars at some point in our life; it's what we lie about that upsets other people or does not upset them, and we can only make an educated guess about whether any particular lie or omission of information will be a problem.