Decided to bother with a reply to this. Our society is so male-centric that a woman who doesn't want a male partner and doesn't perform femininity is counted a not-woman.
I think this has varied over time, but certainly through the span of living memory, it's so ingrained that "Which of them is the man in their relationship?" is seen as a reasonable question. There are also much harsher signals, of course, such as corrective rape as well as physical and verbal assaults. Basically, if you're a woman who visibly rejects every aspect of our male-pleasing role in society, you are not a "valid" woman. But your inferior sex is known, so you aren't a man either. You're something strange and rather unwelcome.
Unless a growing woman has some confident lesbians in her life, there is no social representation of women like her. Media encourage girls to wear makeup, learn hairstyles, follow fashion and be what boys consider desirable. She has no cultural framework by which to understand herself. Faced with the realisation that she is a strange, unwelcome, invalid woman by prevailing standards, she's looking at a difficult and lonely path. It's a lot for a young person to shoulder.
The discovery that she can become valid by transing herself to a semblance of a man must come as a relief, a way out. It promises social acceptance of all that she is, provided she takes the required steps to seem male. That is - by sacrificing her breasts and risking her future health - she can actualise society's prejudice against women like her. As a "man", she'll fit in.
The logical definition of a not-woman is a man. Therefore, it isn't surprising that many people see butch lesbians as equivalent to trans men. And all because society hasn't bothered to embrace the full range of female existence, preferring instead to chuck "masculine" women and "feminine" men aside in adherence to patriarchally determined sex roles.