Having not seen the show, I think that post-coital transphobia usually stems from sexual orientation. "I fucked a girl who used to be a guy, OMG I'M NOT A FAG, BUT NOW I FUCKED A DUDE, *puke.*"

So, Mike, you're right that the reaction was not unheard of or unrealistic. Thinking it's not unrealistic doesn't make you transphobic or homophobic or a terrible person (you're a terrible person because you bust up your evil twin when she's vulnerably post-surgery!). But redstapler is also correct that media portrayal such "realism" contributes towards how people learn such behaviours.