Your premise assumes that if you’re dating a woman and she is, for example, a transwoman who hasn’t or has transitioned, that she’s ‘really a man underneath’. That is incorrect. If you are dating a woman and reading her as a woman and she presents herself as a woman, that’s what she is. If you are unable to get past something body-related or history-related, that’s your problem. Anything else would have to do with crossdressers, I suppose…where you’re dating a woman but that woman identifies as a man who crossdresses. I still can’t see that being common. Oh and just as reminder: many trans people can’t be read by ‘other people’, their gender presentation isn’t ‘suspicious’ and that’s kind of the point, in some ways…they play gender better than cisgender people do.