"Don't" <> "Can't" or "Couldn't".
Emily mis-represented what I said, and criticised me for what she claimed I had said (but didn't).

This place really needs an emoji of Spock doing the eyebrow thing...
You are correct that "don't" != "can't"; however, the circumstances under which you know "You don't" about someone else but you don't know "You can't" about him are a bit limited. So when you claim "You don't", what are we to make of your implied knowledge claim? How do you know he doesn't? Did excreationist PM you and tell you the hypothetical version of himself in his scenario actually doesn't? Do you have the lame doomed-to-be-a-sidekick superpower of extrasensory perception that gives you privileged insight into inner thoughts, but only of fictional characters? Do you have precognition? Did you foresee that excreationist would later admit his character might lie about his preferences, and are you now giving us your supernatural revelation that he is going to confirm that his theoretical self does in fact have a slight attraction to men or dead babies?
All the scenarios in which you know he doesn't but you don't know he can't are absurd. Therefore the only reasonable interpretation of 'You don't prefer "regular* females"' is 'You don't prefer "regular* females" and the way I know that is because you can't.'. That's why Emily evidently inferred you meant that, and then postulated a plausible reason you might believe he can't: the whole "sexual orientation is a figment of everyone's imagination" meme popular with a certain ideology.
So if you insist Emily was unreasonable and "Nothing in what I said could be interpreted to mean that", then by all means, please share with us how you know excreationist's theoretical self doesn't prefer regular females. Your listeners await your explanation with bated breath.