The best I can do: do people with aphantasia only get aroused if the stimulus is present? Can't they not get horny just imagining things, like I imagine most people can?
Does steamy literature do anything for them? I imagine it doesn't, since if you cannot imagine things then words on a page just have no power.
In my opinion, the fact erotic literature exists is proof aphantasia is not normal. Words cannot be arousing if you cannot imagine things "in your mind's eye".